docs: correct Task 4 test move data in the plan

The Task 4 test had East's and West's moves swapped relative to the
PLAYER_BOARDS mapping (E plays NE/SE, W plays NW/SW). Caught and fixed in
the shipped code during execution; this brings the plan document in line.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,7 +1,31 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
 node_modules/
+dist
 dist/
-.superpowers/
+dist-ssr
-.DS_Store
 *.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?
+
+# Project-specific
+.superpowers/
 *.tsbuildinfo
 GITEA_API.md

.secrets.baseline

@@ -0,0 +1,865 @@
+{
+  "version": "1.5.0",
+  "plugins_used": [
+    {
+      "name": "ArtifactoryDetector"
+    },
+    {
+      "name": "AWSKeyDetector"
+    },
+    {
+      "name": "AzureStorageKeyDetector"
+    },
+    {
+      "name": "Base64HighEntropyString",
+      "limit": 4.5
+    },
+    {
+      "name": "BasicAuthDetector"
+    },
+    {
+      "name": "CloudantDetector"
+    },
+    {
+      "name": "DiscordBotTokenDetector"
+    },
+    {
+      "name": "GitHubTokenDetector"
+    },
+    {
+      "name": "GitLabTokenDetector"
+    },
+    {
+      "name": "HexHighEntropyString",
+      "limit": 3.0
+    },
+    {
+      "name": "IbmCloudIamDetector"
+    },
+    {
+      "name": "IbmCosHmacDetector"
+    },
+    {
+      "name": "IPPublicDetector"
+    },
+    {
+      "name": "JwtTokenDetector"
+    },
+    {
+      "name": "KeywordDetector",
+      "keyword_exclude": ""
+    },
+    {
+      "name": "MailchimpDetector"
+    },
+    {
+      "name": "NpmDetector"
+    },
+    {
+      "name": "OpenAIDetector"
+    },
+    {
+      "name": "PrivateKeyDetector"
+    },
+    {
+      "name": "PypiTokenDetector"
+    },
+    {
+      "name": "SendGridDetector"
+    },
+    {
+      "name": "SlackDetector"
+    },
+    {
+      "name": "SoftlayerDetector"
+    },
+    {
+      "name": "SquareOAuthDetector"
+    },
+    {
+      "name": "StripeDetector"
+    },
+    {
+      "name": "TelegramBotTokenDetector"
+    },
+    {
+      "name": "TwilioKeyDetector"
+    }
+  ],
+  "filters_used": [
+    {
+      "path": "detect_secrets.filters.allowlist.is_line_allowlisted"
+    },
+    {
+      "path": "detect_secrets.filters.common.is_ignored_due_to_verification_policies",
+      "min_level": 2
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_indirect_reference"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_likely_id_string"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_lock_file"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_not_alphanumeric_string"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_potential_uuid"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_prefixed_with_dollar_sign"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_sequential_string"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_swagger_file"
+    },
+    {
+      "path": "detect_secrets.filters.heuristic.is_templated_secret"
+    }
+  ],
+  "results": {
+    "pnpm-lock.yaml": [
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "88dcf5e48940431d207eb6beeda3f28b52b9dcd6",
+        "is_verified": false,
+        "line_number": 43
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "9b1aeb3238bb5411805279e20afb4fb96fa7490d",
+        "is_verified": false,
+        "line_number": 46
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "e8cc5935afdfd3788eae945e3db3f42e2c3acee6",
+        "is_verified": false,
+        "line_number": 49
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "8733ba82d33885a92ca3009eb6ba61e505a1ee9a",
+        "is_verified": false,
+        "line_number": 52
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "3e3522c83d724aa4133ee6bb6d627e4390be31a5",
+        "is_verified": false,
+        "line_number": 55
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "9d1b9dbec9f048e048f4dd38591947ca7deff5dd",
+        "is_verified": false,
+        "line_number": 58
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "f60c00cacddf514ad090fbde7005d8721adc0b89",
+        "is_verified": false,
+        "line_number": 62
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "b0e9cbf320ebe74507bcdaae3f0b40740459a092",
+        "is_verified": false,
+        "line_number": 65
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "f274e993f5da6ef04f9ab21f5152b7f16f86fc45",
+        "is_verified": false,
+        "line_number": 68
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "63ec2819c4e19e52c1d683e5e3e2e6ce0b6cac93",
+        "is_verified": false,
+        "line_number": 74
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "907c22434e5fe282d088f338a9e6155fb51ce241",
+        "is_verified": false,
+        "line_number": 77
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "e694aeb21a36497b6eb9bc56f884bd4c69ac5add",
+        "is_verified": false,
+        "line_number": 83
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "4d2585f3339105214d7685879b53a916f5aea9fa",
+        "is_verified": false,
+        "line_number": 89
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "f1833f6e7490c05a1a76205a4bdafb80e88e04e6",
+        "is_verified": false,
+        "line_number": 95
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "cacf9ecad80e93e92e88556952a24dedeae071ae",
+        "is_verified": false,
+        "line_number": 101
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "525571cc69c07ea83a7f575ab13f8174a1279317",
+        "is_verified": false,
+        "line_number": 107
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "be823e3374be22151f74deb53ad9b52ba62e1794",
+        "is_verified": false,
+        "line_number": 114
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "f057529a66a11ba122564fc9c36ccefe29df6f1a",
+        "is_verified": false,
+        "line_number": 121
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "2e065f3f711d422e5ef5abdcd9c255dea44113f7",
+        "is_verified": false,
+        "line_number": 128
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "04eddfeea8ec2c47eac1fdc1f6033f88d175e4cf",
+        "is_verified": false,
+        "line_number": 135
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "de1fe5a83dde4beebb24a15f2ebf259aa1adb023",
+        "is_verified": false,
+        "line_number": 142
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "aaa92f5d670cc00ba757939cebeda4c34436c037",
+        "is_verified": false,
+        "line_number": 149
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "2cd4187cd1381a5ff2776451a79449d69a7790ec",
+        "is_verified": false,
+        "line_number": 155
+      },
+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "a427b668faefb136b19160d59226685364b3a9e5",
+        "is_verified": false,
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+      {
+        "type": "Base64 High Entropy String",
+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "007e6990046ea7fce8af0547e80941651f78eb17",
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+      {
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+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "b0da61692236cf0f51dfca9bea5b058e6db2a173",
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+      {
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+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "1e9ff9fee5ad181d3f63618e0267e8ad42826e83",
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+      {
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+        "filename": "pnpm-lock.yaml",
+        "hashed_secret": "0e08e31e43853476fc57d10643d8c871082c08b3",
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+      {
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+        "filename": "pnpm-lock.yaml",
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+      {
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+  "generated_at": "2026-05-19T04:40:00Z"
+}

.vscode/extensions.json

@@ -0,0 +1,3 @@
+{
+  "recommendations": ["svelte.svelte-vscode"]
+}

README.md

@@ -0,0 +1,30 @@
+# Duplicate Chess
+
+A local, single-operator browser sandbox for **duplicate chess** — a four-player
+chess variant invented by Andrew Freiberg. Four players (North, South, East, West)
+and four boards (NW, NE, SW, SE); each player controls one colour on two boards and
+must play the identical move on both. A captured piece leaves a frozen "ghost" twin
+on the player's other board.
+
+This tool puts all four boards on one screen as a rotated "compass", enforces the
+synchronized-move coupling, renders ghosts, shows the move-legality intersection,
+and detects the endgame.
+
+## Develop
+
+```
+pnpm install
+pnpm dev      # run the dev server
+pnpm build    # production build
+pnpm test     # run the engine test suite (vitest)
+```
+
+`pnpm exec svelte-check --tsconfig ./tsconfig.json` typechecks the Svelte/TS code.
+
+## Design
+
+See `docs/superpowers/specs/2026-05-19-duplicate-chess-design.md` for the full design,
+and `docs/superpowers/plans/2026-05-19-duplicate-chess-sandbox.md` for the build plan.
+
+This is v1 — a local sandbox. Networked multiplayer, AI opponents, and a free
+position editor are explicitly out of scope (see the spec, §7).

docs/superpowers/plans/2026-05-19-duplicate-chess-sandbox.md

@@ -496,12 +496,12 @@ describe('legalSyncedMoves', () => {
   });
 
   it('excludes a move legal on only one of the player\'s boards', () => {
-    // N e2e4, S e2e4, E e7e5, W d7d5  -> NW black has pe5, NE black has pd5.
+    // N e2e4, S e2e4, E d7d5, W e7e5  -> NW black has pe5 (W's), NE black has pd5 (E's).
     const g = new DuplicateGame([
       { player: 'N', from: 'e2', to: 'e4' },
       { player: 'S', from: 'e2', to: 'e4' },
-      { player: 'E', from: 'e7', to: 'e5' },
+      { player: 'E', from: 'd7', to: 'd5' },
-      { player: 'W', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
     ]);
     expect(g.currentPlayer).toBe('N');
     const keys = legalSyncedMoves(g).map((m) => `${m.from}${m.to}`);
@@ -525,8 +525,8 @@ describe('selectionHighlight', () => {
     const g = new DuplicateGame([
       { player: 'N', from: 'e2', to: 'e4' },
       { player: 'S', from: 'e2', to: 'e4' },
-      { player: 'E', from: 'e7', to: 'e5' },
+      { player: 'E', from: 'd7', to: 'd5' },
-      { player: 'W', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
     ]);
     const h = selectionHighlight(g, 'e4'); // North's e4 pawn
     expect(h.boardA).toBe('NW');

index.html

@@ -0,0 +1,13 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>vite-tmp</title>
+  </head>
+  <body>
+    <div id="app"></div>
+    <script type="module" src="/src/main.ts"></script>
+  </body>
+</html>

package.json

@@ -0,0 +1,27 @@
+{
+  "name": "duplicate-chess",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview",
+    "check": "svelte-check --tsconfig ./tsconfig.app.json && tsc -p tsconfig.node.json",
+    "test": "vitest run --passWithNoTests",
+    "test:watch": "vitest"
+  },
+  "devDependencies": {
+    "@sveltejs/vite-plugin-svelte": "^7.1.2",
+    "@tsconfig/svelte": "^5.0.8",
+    "@types/node": "^24.12.3",
+    "svelte": "^5.55.5",
+    "svelte-check": "^4.4.8",
+    "typescript": "~6.0.2",
+    "vite": "^8.0.12",
+    "vitest": "^4.1.6"
+  },
+  "dependencies": {
+    "chess.js": "^1.4.0"
+  }
+}

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+  <symbol id="x-icon" viewBox="0 0 19 19">
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src/App.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+  import Compass from './lib/Compass.svelte';
+  import Panel from './lib/Panel.svelte';
+  import PromotionDialog from './lib/PromotionDialog.svelte';
+</script>
+
+<header>
+  <h1>Duplicate Chess</h1>
+  <p>Local sandbox — operator drives all four players. Click a piece on the
+     glowing boards to see its synchronized-legal moves.</p>
+</header>
+
+<main>
+  <Compass />
+  <Panel />
+</main>
+
+<PromotionDialog />
+
+<style>
+  header { padding: 14px 22px; border-bottom: 1px solid #333845; }
+  header h1 { margin: 0; font-size: 17px; }
+  header p { margin: 4px 0 0; font-size: 12px; color: #9aa0aa; }
+  main { display: flex; gap: 22px; padding: 20px 22px; align-items: flex-start; }
+</style>

src/app.css

@@ -0,0 +1,8 @@
+:root { color-scheme: dark; }
+* { box-sizing: border-box; }
+body {
+  margin: 0;
+  background: #15171c;
+  color: #e6e8ec;
+  font-family: -apple-system, "Segoe UI", Roboto, sans-serif;
+}

src/engine/boards.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from 'vitest';
+import { BOARD_IDS, PLAYERS, PLAYER_BOARDS, PLAYER_COLOR, BOARD_PLAYERS, BOARD_ROTATION } from './boards';
+
+describe('boards constants', () => {
+  it('lists four boards and four players in turn order', () => {
+    expect(BOARD_IDS).toEqual(['NW', 'NE', 'SW', 'SE']);
+    expect(PLAYERS).toEqual(['N', 'S', 'E', 'W']);
+  });
+
+  it('each player controls exactly two boards', () => {
+    for (const p of PLAYERS) expect(PLAYER_BOARDS[p]).toHaveLength(2);
+    expect(PLAYER_BOARDS.N).toEqual(['NW', 'NE']);
+    expect(PLAYER_BOARDS.W).toEqual(['NW', 'SW']);
+  });
+
+  it('board players are consistent with player boards', () => {
+    for (const b of BOARD_IDS) {
+      const { w, b: black } = BOARD_PLAYERS[b];
+      expect(PLAYER_BOARDS[w]).toContain(b);
+      expect(PLAYER_BOARDS[black]).toContain(b);
+      expect(PLAYER_COLOR[w]).toBe('w');
+      expect(PLAYER_COLOR[black]).toBe('b');
+    }
+  });
+
+  it('has a rotation for every board', () => {
+    expect(BOARD_ROTATION).toEqual({ NW: 225, NE: 135, SW: 315, SE: 45 });
+  });
+});

src/engine/boards.ts

@@ -0,0 +1,32 @@
+import type { BoardId, Player, Color } from './types';
+
+export const BOARD_IDS: BoardId[] = ['NW', 'NE', 'SW', 'SE'];
+
+/** Turn order. */
+export const PLAYERS: Player[] = ['N', 'S', 'E', 'W'];
+
+/** The two boards each player controls (order is stable: [boardA, boardB]). */
+export const PLAYER_BOARDS: Record<Player, [BoardId, BoardId]> = {
+  N: ['NW', 'NE'],
+  S: ['SW', 'SE'],
+  E: ['NE', 'SE'],
+  W: ['NW', 'SW'],
+};
+
+/** The colour each player plays on both their boards. */
+export const PLAYER_COLOR: Record<Player, Color> = {
+  N: 'w', S: 'w', E: 'b', W: 'b',
+};
+
+/** The white and black player of each board. */
+export const BOARD_PLAYERS: Record<BoardId, { w: Player; b: Player }> = {
+  NW: { w: 'N', b: 'W' },
+  NE: { w: 'N', b: 'E' },
+  SW: { w: 'S', b: 'W' },
+  SE: { w: 'S', b: 'E' },
+};
+
+/** Compass rotation in degrees for rendering each board (see spec §5.1). */
+export const BOARD_ROTATION: Record<BoardId, number> = {
+  NW: 225, NE: 135, SW: 315, SE: 45,
+};

src/engine/endgame.test.ts

@@ -0,0 +1,61 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { playSymmetric } from './test-helpers';
+import { evaluateStatus } from './endgame';
+
+describe('evaluateStatus', () => {
+  it('reports an ongoing game at the start', () => {
+    const s = evaluateStatus(new DuplicateGame());
+    expect(s.state).toBe('playing');
+    expect(s.checks).toEqual([]);
+  });
+
+  it('detects a double-board checkmate (Fool\'s mate, played symmetrically)', () => {
+    const g = new DuplicateGame();
+    playSymmetric(g, [
+      [{ from: 'f2', to: 'f3' }, { from: 'e7', to: 'e5' }],
+      [{ from: 'g2', to: 'g4' }, { from: 'd8', to: 'h4' }],
+    ]);
+    expect(g.currentPlayer).toBe('N'); // North (White) is mated
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('checkmate');
+    expect(s.checks.sort()).toEqual(['NE', 'NW']);
+    expect(s.result).toEqual({ N: 'loss', S: 'draw', E: 'win', W: 'win' });
+  });
+
+  it('detects threefold repetition of the whole system', () => {
+    const g = new DuplicateGame();
+    const cycle: Array<[{ from: string; to: string }, { from: string; to: string }]> = [
+      [{ from: 'g1', to: 'f3' }, { from: 'g8', to: 'f6' }],
+      [{ from: 'f3', to: 'g1' }, { from: 'f6', to: 'g8' }],
+    ];
+    playSymmetric(g, cycle); // back to start (occurrence 2)
+    playSymmetric(g, cycle); // back to start (occurrence 3)
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('draw');
+    expect(s.reason).toBe('threefold');
+    expect(s.result).toEqual({ N: 'draw', S: 'draw', E: 'draw', W: 'draw' });
+  });
+
+  it('detects a stalemate as an all-draw game end (provisional rule)', () => {
+    const g = new DuplicateGame();
+    // The known fastest stalemate, played symmetrically on all four boards.
+    playSymmetric(g, [
+      [{ from: 'e2', to: 'e3' }, { from: 'a7', to: 'a5' }],
+      [{ from: 'd1', to: 'h5' }, { from: 'a8', to: 'a6' }],
+      [{ from: 'h5', to: 'a5' }, { from: 'h7', to: 'h5' }],
+      [{ from: 'a5', to: 'c7' }, { from: 'a6', to: 'h6' }],
+      [{ from: 'h2', to: 'h4' }, { from: 'f7', to: 'f6' }],
+      [{ from: 'c7', to: 'd7' }, { from: 'e8', to: 'f7' }],
+      [{ from: 'd7', to: 'b7' }, { from: 'd8', to: 'd3' }],
+      [{ from: 'b7', to: 'b8' }, { from: 'd3', to: 'h7' }],
+      [{ from: 'b8', to: 'c8' }, { from: 'f7', to: 'g6' }],
+      [{ from: 'c8', to: 'e6' }], // no black reply — Black is stalemated
+    ]);
+    expect(g.currentPlayer).toBe('E'); // a Black player, with no move
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('stalemate');
+    expect(s.reason).toBe('stalemate');
+    expect(s.result).toEqual({ N: 'draw', S: 'draw', E: 'draw', W: 'draw' });
+  });
+});

src/engine/endgame.ts

@@ -0,0 +1,51 @@
+import type { DuplicateGame } from './game';
+import type { GameStatus, GameResult, BoardId } from './types';
+import { PLAYERS, PLAYER_BOARDS, BOARD_PLAYERS } from './boards';
+import { legalSyncedMoves } from './legality';
+
+/** PROVISIONAL (spec §6): the 50-move rule fires after this many rounds. */
+const FIFTY_MOVE_ROUNDS = 50;
+const FIFTY_MOVE_PLIES = FIFTY_MOVE_ROUNDS * 4;
+
+function allDraw(): GameResult {
+  return { N: 'draw', S: 'draw', E: 'draw', W: 'draw' };
+}
+
+/** Evaluate the game from the perspective of the player to move. */
+export function evaluateStatus(game: DuplicateGame): GameStatus {
+  const player = game.currentPlayer;
+  const [a, b] = PLAYER_BOARDS[player];
+
+  const checks: BoardId[] = [];
+  if (game.boards[a].inCheck()) checks.push(a);
+  if (game.boards[b].inCheck()) checks.push(b);
+
+  const synced = legalSyncedMoves(game);
+
+  if (synced.length === 0) {
+    if (checks.length > 0) {
+      // Checkmate. PROVISIONAL (spec §6): every opponent delivering a check wins.
+      const winners = checks.map((board) =>
+        BOARD_PLAYERS[board].w === player
+          ? BOARD_PLAYERS[board].b
+          : BOARD_PLAYERS[board].w,
+      );
+      const result = {} as GameResult;
+      for (const p of PLAYERS) {
+        result[p] = p === player ? 'loss' : winners.includes(p) ? 'win' : 'draw';
+      }
+      return { state: 'checkmate', result, checks };
+    }
+    // PROVISIONAL (spec §6): a no-synchronized-move stalemate ends the game, all draw.
+    return { state: 'stalemate', result: allDraw(), reason: 'stalemate', checks };
+  }
+
+  if (game.repetitionCount() >= 3) {
+    return { state: 'draw', result: allDraw(), reason: 'threefold', checks };
+  }
+  if (game.pliesSinceProgress >= FIFTY_MOVE_PLIES) {
+    return { state: 'draw', result: allDraw(), reason: 'fifty-move', checks };
+  }
+
+  return { state: 'playing', checks };
+}

src/engine/game.test.ts

@@ -0,0 +1,73 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+
+const START = 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR';
+
+function placement(fen: string) {
+  return fen.split(' ')[0];
+}
+
+describe('DuplicateGame', () => {
+  it('starts with four boards in the standard position, North to move', () => {
+    const g = new DuplicateGame();
+    expect(g.ply).toBe(0);
+    expect(g.currentPlayer).toBe('N');
+    for (const id of ['NW', 'NE', 'SW', 'SE'] as const) {
+      expect(placement(g.boards[id].fen())).toBe(START);
+    }
+  });
+
+  it("applies a synchronized move to the current player's two boards only", () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // North
+    expect(g.ply).toBe(1);
+    expect(g.currentPlayer).toBe('S');
+    expect(placement(g.boards.NW.fen())).toContain('4P3'); // pawn advanced
+    expect(placement(g.boards.NE.fen())).toContain('4P3');
+    expect(placement(g.boards.SW.fen())).toBe(START); // untouched
+    expect(placement(g.boards.SE.fen())).toBe(START);
+  });
+
+  it('cycles the current player N -> S -> E -> W -> N', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // N
+    g.applyMove({ from: 'e2', to: 'e4' }); // S
+    g.applyMove({ from: 'e7', to: 'e5' }); // E
+    g.applyMove({ from: 'e7', to: 'e5' }); // W
+    expect(g.currentPlayer).toBe('N');
+    expect(g.ply).toBe(4);
+  });
+
+  it("throws on a move not legal on both of the player's boards", () => {
+    const g = new DuplicateGame();
+    expect(() => g.applyMove({ from: 'e2', to: 'e5' })).toThrow();
+  });
+
+  it('undo removes the last move and restores the position', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' });
+    g.undo();
+    expect(g.ply).toBe(0);
+    expect(g.currentPlayer).toBe('N');
+    expect(placement(g.boards.NW.fen())).toBe(START);
+  });
+
+  it('rebuilds from a history array passed to the constructor', () => {
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+    ]);
+    expect(g.ply).toBe(2);
+    expect(g.currentPlayer).toBe('E');
+  });
+
+  it('resets the progress clock on a pawn move or capture', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // pawn move -> clock stays 0
+    expect(g.pliesSinceProgress).toBe(0);
+    g.applyMove({ from: 'e2', to: 'e4' }); // pawn move
+    expect(g.pliesSinceProgress).toBe(0);
+    g.applyMove({ from: 'g8', to: 'f6' }); // knight move -> clock increments
+    expect(g.pliesSinceProgress).toBe(1);
+  });
+});

src/engine/game.ts

@@ -0,0 +1,93 @@
+import { Chess } from 'chess.js';
+import type { BoardId, Player, SyncMove, HistoryEntry } from './types';
+import { BOARD_IDS, PLAYERS, PLAYER_BOARDS } from './boards';
+
+/** A chess.js move result has these fields we rely on. */
+interface ChessMoveResult {
+  piece: string;
+  captured?: string;
+}
+
+export class DuplicateGame {
+  readonly boards: Record<BoardId, Chess>;
+  readonly history: HistoryEntry[] = [];
+
+  /** Plies since the last capture or pawn move on any board (for the 50-move rule). */
+  pliesSinceProgress = 0;
+  /** Repetition keys of the whole 4-board system, one per position incl. the start. */
+  readonly repetitionKeys: string[] = [];
+
+  constructor(history: HistoryEntry[] = []) {
+    this.boards = {
+      NW: new Chess(), NE: new Chess(), SW: new Chess(), SE: new Chess(),
+    };
+    this.repetitionKeys.push(this.systemKey());
+    for (const entry of history) this.applyMove(entry, entry.player);
+  }
+
+  get ply(): number {
+    return this.history.length;
+  }
+
+  get currentPlayer(): Player {
+    return PLAYERS[this.history.length % 4];
+  }
+
+  /** Whether `move` is a legal chess move on `board` in the current position. */
+  isLegalOnBoard(board: BoardId, move: SyncMove): boolean {
+    return this.boards[board].moves({ verbose: true }).some(
+      (m) =>
+        m.from === move.from &&
+        m.to === move.to &&
+        (m.promotion ?? undefined) === (move.promotion ?? undefined),
+    );
+  }
+
+  /** Apply one synchronized move to a player's two boards. Throws if illegal on either. */
+  applyMove(move: SyncMove, player: Player = this.currentPlayer): void {
+    const [a, b] = PLAYER_BOARDS[player];
+    if (!this.isLegalOnBoard(a, move) || !this.isLegalOnBoard(b, move)) {
+      throw new Error(
+        `Illegal synchronized move ${move.from}${move.to} for ${player}`,
+      );
+    }
+    const ra = this.boards[a].move(move) as unknown as ChessMoveResult;
+    const rb = this.boards[b].move(move) as unknown as ChessMoveResult;
+    this.history.push({ ...move, player });
+    const progress = isProgress(ra) || isProgress(rb);
+    this.pliesSinceProgress = progress ? 0 : this.pliesSinceProgress + 1;
+    this.repetitionKeys.push(this.systemKey());
+  }
+
+  /** Remove the last move by replaying the truncated history. */
+  undo(): void {
+    if (this.history.length === 0) return;
+    const replay = this.history.slice(0, -1);
+    for (const id of BOARD_IDS) this.boards[id].reset();
+    this.history.length = 0;
+    this.pliesSinceProgress = 0;
+    this.repetitionKeys.length = 0;
+    this.repetitionKeys.push(this.systemKey());
+    for (const e of replay) this.applyMove(e, e.player);
+  }
+
+  /** A repetition key for the whole system: each board's placement+castling+ep, plus the side to move. */
+  systemKey(): string {
+    return (
+      BOARD_IDS.map((id) => {
+        const parts = this.boards[id].fen().split(' ');
+        return `${parts[0]}${parts[2]}${parts[3]}`;
+      }).join('|') + ':' + this.currentPlayer
+    );
+  }
+
+  /** How many times the current system position has occurred. */
+  repetitionCount(): number {
+    const current = this.repetitionKeys[this.repetitionKeys.length - 1];
+    return this.repetitionKeys.filter((k) => k === current).length;
+  }
+}
+
+function isProgress(result: ChessMoveResult): boolean {
+  return result.piece === 'p' || result.captured != null;
+}

src/engine/ghosts.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { playSymmetric } from './test-helpers';
+import { ghosts } from './ghosts';
+
+describe('ghosts', () => {
+  it('reports no ghosts at the start', () => {
+    expect(ghosts(new DuplicateGame())).toEqual([]);
+  });
+
+  it('forms a ghost when a piece is captured on one board but not its twin', () => {
+    const g = new DuplicateGame();
+    // Symmetric opening so all four boards stay identical...
+    playSymmetric(g, [
+      [{ from: 'e2', to: 'e4' }, { from: 'e7', to: 'e5' }],
+      [{ from: 'g1', to: 'f3' }, { from: 'b8', to: 'c6' }],
+    ]);
+    // ...then North & South each play Nxe5 (capturing the e5 pawn),
+    // and East captures that knight only on its boards (NE, SE).
+    g.applyMove({ from: 'f3', to: 'e5' }); // N: Nf3xe5 on NW, NE
+    g.applyMove({ from: 'f3', to: 'e5' }); // S: Nf3xe5 on SW, SE
+    g.applyMove({ from: 'c6', to: 'e5' }); // E: Nc6xe5 on NE, SE — captures the white knight
+    // North's knight on e5 survives on NW but was captured on NE -> NW/e5 is a ghost.
+    // South's knight on e5 survives on SW but was captured on SE -> SW/e5 is a ghost.
+    const result = ghosts(g).sort((x, y) => (x.board < y.board ? -1 : 1));
+    expect(result).toEqual([
+      { board: 'NW', square: 'e5' },
+      { board: 'SW', square: 'e5' },
+    ]);
+  });
+});

src/engine/ghosts.ts

@@ -0,0 +1,32 @@
+import type { DuplicateGame } from './game';
+import type { GhostMarker, BoardId, Color, Square } from './types';
+import { PLAYERS, PLAYER_BOARDS, PLAYER_COLOR } from './boards';
+
+/** Squares occupied by a piece of `color` on `board`. */
+function colorSquares(game: DuplicateGame, board: BoardId, color: Color): Set<Square> {
+  const set = new Set<Square>();
+  for (const row of game.boards[board].board()) {
+    for (const cell of row) {
+      if (cell && cell.color === color) set.add(cell.square);
+    }
+  }
+  return set;
+}
+
+/**
+ * Ghosts across all four players. A player's non-ghost pieces always occupy
+ * identical squares on both their boards (they move in lockstep), so a piece is
+ * a ghost iff the player's other board has no same-colour piece on that square.
+ */
+export function ghosts(game: DuplicateGame): GhostMarker[] {
+  const markers: GhostMarker[] = [];
+  for (const player of PLAYERS) {
+    const [a, b] = PLAYER_BOARDS[player];
+    const color = PLAYER_COLOR[player];
+    const sqA = colorSquares(game, a, color);
+    const sqB = colorSquares(game, b, color);
+    for (const sq of sqA) if (!sqB.has(sq)) markers.push({ board: a, square: sq });
+    for (const sq of sqB) if (!sqA.has(sq)) markers.push({ board: b, square: sq });
+  }
+  return markers;
+}

src/engine/integration.test.ts

@@ -0,0 +1,47 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { playSymmetric } from './test-helpers';
+import { legalSyncedMoves } from './legality';
+import { ghosts } from './ghosts';
+import { evaluateStatus } from './endgame';
+import { serialize, deserialize } from './notation';
+
+describe('integration: a scripted game played to checkmate', () => {
+  it('plays Fool\'s mate, stays consistent throughout, and ends correctly', () => {
+    const g = new DuplicateGame();
+
+    // The game is live and ongoing until the mate.
+    expect(evaluateStatus(g).state).toBe('playing');
+    expect(legalSyncedMoves(g).length).toBeGreaterThan(0);
+
+    playSymmetric(g, [
+      [{ from: 'f2', to: 'f3' }, { from: 'e7', to: 'e5' }],
+      [{ from: 'g2', to: 'g4' }, { from: 'd8', to: 'h4' }],
+    ]);
+
+    // No captures occurred, so there are no ghosts.
+    expect(ghosts(g)).toEqual([]);
+
+    // North is checkmated.
+    const status = evaluateStatus(g);
+    expect(status.state).toBe('checkmate');
+    expect(status.result?.N).toBe('loss');
+    expect(legalSyncedMoves(g)).toEqual([]);
+
+    // The game round-trips through save/load and reproduces the same outcome.
+    const restored = new DuplicateGame(deserialize(serialize(g.history)));
+    expect(evaluateStatus(restored).state).toBe('checkmate');
+    expect(restored.history).toEqual(g.history);
+  });
+
+  it('undo from the mated position restores a playable game', () => {
+    const g = new DuplicateGame();
+    playSymmetric(g, [
+      [{ from: 'f2', to: 'f3' }, { from: 'e7', to: 'e5' }],
+      [{ from: 'g2', to: 'g4' }, { from: 'd8', to: 'h4' }],
+    ]);
+    g.undo(); // take back W's d8h4
+    expect(evaluateStatus(g).state).toBe('playing');
+    expect(g.currentPlayer).toBe('W');
+  });
+});

src/engine/legality.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { legalSyncedMoves, selectionHighlight } from './legality';
+
+describe('legalSyncedMoves', () => {
+  it('returns all 20 white opening moves when a player\'s two boards are identical', () => {
+    const g = new DuplicateGame();
+    expect(legalSyncedMoves(g)).toHaveLength(20);
+  });
+
+  it('excludes a move legal on only one of the player\'s boards', () => {
+    // N e2e4, S e2e4, W e7e5, E d7d5  -> NW black has pe5, NE black has pd5.
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+      { player: 'E', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
+    ]);
+    expect(g.currentPlayer).toBe('N');
+    const keys = legalSyncedMoves(g).map((m) => `${m.from}${m.to}`);
+    // e4-d5 is a capture on NE but illegal on NW (d5 empty there): not synced.
+    expect(keys).not.toContain('e4d5');
+    // e4-e5 is legal on NE but blocked on NW (black pawn on e5): not synced.
+    expect(keys).not.toContain('e4e5');
+  });
+});
+
+describe('selectionHighlight', () => {
+  it('marks every destination playable when the two boards agree', () => {
+    const g = new DuplicateGame();
+    const h = selectionHighlight(g, 'e2');
+    expect(h.playable.sort()).toEqual(['e3', 'e4']);
+    expect(h.onlyA).toEqual([]);
+    expect(h.onlyB).toEqual([]);
+  });
+
+  it('splits destinations into playable vs board-local-only on divergence', () => {
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+      { player: 'E', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
+    ]);
+    const h = selectionHighlight(g, 'e4'); // North's e4 pawn
+    expect(h.boardA).toBe('NW');
+    expect(h.boardB).toBe('NE');
+    expect(h.playable).toEqual([]);          // nothing legal on both
+    expect(h.onlyA).toEqual([]);             // e4 is blocked on NW
+    expect(h.onlyB.sort()).toEqual(['d5', 'e5']); // capture + advance on NE only
+  });
+});

src/engine/legality.ts

@@ -0,0 +1,61 @@
+import type { DuplicateGame } from './game';
+import type { SyncMove, Square, BoardId, PromotionPiece } from './types';
+import { PLAYER_BOARDS } from './boards';
+
+function key(m: { from: string; to: string; promotion?: string }): string {
+  return `${m.from}${m.to}${m.promotion ?? ''}`;
+}
+
+/** Every synchronized-legal move for the player to move (the intersection). */
+export function legalSyncedMoves(game: DuplicateGame): SyncMove[] {
+  const [a, b] = PLAYER_BOARDS[game.currentPlayer];
+  const movesA = game.boards[a].moves({ verbose: true });
+  const keysB = new Set(game.boards[b].moves({ verbose: true }).map(key));
+  const seen = new Set<string>();
+  const result: SyncMove[] = [];
+  for (const m of movesA) {
+    const k = key(m);
+    if (keysB.has(k) && !seen.has(k)) {
+      seen.add(k);
+      result.push({
+        from: m.from,
+        to: m.to,
+        promotion: (m.promotion as PromotionPiece) || undefined,
+      });
+    }
+  }
+  return result;
+}
+
+export interface SelectionHighlight {
+  /** The current player's first board. */
+  boardA: BoardId;
+  /** The current player's second board. */
+  boardB: BoardId;
+  /** Destinations legal on BOTH boards (actually playable). */
+  playable: Square[];
+  /** Destinations legal on board A only. */
+  onlyA: Square[];
+  /** Destinations legal on board B only. */
+  onlyB: Square[];
+}
+
+/** Triple-highlight data for the current player's piece grabbed at `from`. */
+export function selectionHighlight(
+  game: DuplicateGame,
+  from: Square,
+): SelectionHighlight {
+  const [a, b] = PLAYER_BOARDS[game.currentPlayer];
+  const destA = new Set(
+    game.boards[a].moves({ verbose: true }).filter((m) => m.from === from).map((m) => m.to),
+  );
+  const destB = new Set(
+    game.boards[b].moves({ verbose: true }).filter((m) => m.from === from).map((m) => m.to),
+  );
+  const playable: Square[] = [];
+  const onlyA: Square[] = [];
+  const onlyB: Square[] = [];
+  for (const sq of destA) (destB.has(sq) ? playable : onlyA).push(sq);
+  for (const sq of destB) if (!destA.has(sq)) onlyB.push(sq);
+  return { boardA: a, boardB: b, playable, onlyA, onlyB };
+}

src/engine/notation.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest';
+import { toCoordinate, serialize, deserialize } from './notation';
+import type { HistoryEntry } from './types';
+
+describe('notation', () => {
+  it('renders a move as a coordinate token', () => {
+    expect(toCoordinate({ from: 'e2', to: 'e4' })).toBe('e2e4');
+    expect(toCoordinate({ from: 'e7', to: 'e8', promotion: 'q' })).toBe('e7e8q');
+  });
+
+  it('round-trips a game through serialize/deserialize', () => {
+    const history: HistoryEntry[] = [
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+    ];
+    const restored = deserialize(serialize(history));
+    expect(restored).toEqual(history);
+  });
+
+  it('rejects a file that is not a duplicate-chess save', () => {
+    expect(() => deserialize('{"variant":"chess","version":1,"moves":[]}')).toThrow();
+  });
+
+  it('rejects an unsupported save version', () => {
+    expect(() => deserialize('{"variant":"duplicate-chess","version":99,"moves":[]}')).toThrow();
+  });
+});

src/engine/notation.ts

@@ -0,0 +1,28 @@
+import type { SyncMove, HistoryEntry } from './types';
+
+/** Coordinate notation, e.g. "e2e4" or "e7e8q". */
+export function toCoordinate(move: SyncMove): string {
+  return `${move.from}${move.to}${move.promotion ?? ''}`;
+}
+
+export interface SavedGame {
+  variant: 'duplicate-chess';
+  version: 1;
+  moves: HistoryEntry[];
+}
+
+export function serialize(history: HistoryEntry[]): string {
+  const data: SavedGame = { variant: 'duplicate-chess', version: 1, moves: history };
+  return JSON.stringify(data, null, 2);
+}
+
+export function deserialize(json: string): HistoryEntry[] {
+  const data = JSON.parse(json) as Partial<SavedGame>;
+  if (data.variant !== 'duplicate-chess' || !Array.isArray(data.moves)) {
+    throw new Error('Not a duplicate-chess save file');
+  }
+  if (data.version !== 1) {
+    throw new Error('Unsupported save version');
+  }
+  return data.moves;
+}

src/engine/test-helpers.ts

@@ -0,0 +1,23 @@
+import type { DuplicateGame } from './game';
+import type { SyncMove } from './types';
+
+/**
+ * Apply a list of [whiteMove, blackMove?] underlying ply-pairs symmetrically:
+ * North then South play the white move, East then West play the black move.
+ * While every move is symmetric all four boards stay identical, so each board
+ * behaves as an ordinary chess game — useful for reaching ordinary checkmate /
+ * stalemate / repetition positions. Omit blackMove for a final unanswered white move.
+ */
+export function playSymmetric(
+  game: DuplicateGame,
+  pairs: Array<[SyncMove, SyncMove?]>,
+): void {
+  for (const [white, black] of pairs) {
+    game.applyMove(white); // N
+    game.applyMove(white); // S
+    if (black) {
+      game.applyMove(black); // E
+      game.applyMove(black); // W
+    }
+  }
+}

src/engine/types.ts

@@ -0,0 +1,35 @@
+export type BoardId = 'NW' | 'NE' | 'SW' | 'SE';
+export type Player = 'N' | 'S' | 'E' | 'W';
+export type Color = 'w' | 'b';
+export type Square = string;
+export type PromotionPiece = 'q' | 'r' | 'b' | 'n';
+
+export interface SyncMove {
+  from: Square;
+  to: Square;
+  promotion?: PromotionPiece;
+}
+
+export interface HistoryEntry extends SyncMove {
+  player: Player;
+}
+
+export interface GhostMarker {
+  board: BoardId;
+  square: Square;
+}
+
+export type PlayerResult = 'win' | 'draw' | 'loss';
+export type GameResult = Record<Player, PlayerResult>;
+export type GameState = 'playing' | 'checkmate' | 'stalemate' | 'draw';
+export type DrawReason = 'stalemate' | 'threefold' | 'fifty-move' | 'manual';
+
+export interface GameStatus {
+  state: GameState;
+  /** Present when state !== 'playing'. */
+  result?: GameResult;
+  /** Present for a draw/stalemate. */
+  reason?: DrawReason;
+  /** Boards on which the player to move is currently in check. */
+  checks: BoardId[];
+}

src/lib/Board.svelte

@@ -0,0 +1,112 @@
+<script lang="ts">
+  import type { BoardId, Player, Square } from '../engine/types';
+  import type { SelectionHighlight } from '../engine/legality';
+  import { BOARD_ROTATION, BOARD_PLAYERS } from '../engine/boards';
+
+  interface Props {
+    id: BoardId;
+    fen: string;
+    /** Player colours, e.g. { N:'#4a90d9', ... }. */
+    colors: Record<Player, string>;
+    ghosts: Square[];
+    /** Highlight for this board, or null if no piece is grabbed / not active. */
+    highlight: { playable: Square[]; local: Square[]; selected: Square | null } | null;
+    active: boolean;
+    onSquare: (square: Square) => void;
+  }
+
+  let { id, fen, colors, ghosts, highlight, active, onSquare }: Props = $props();
+
+  const FILES = 'abcdefgh';
+  const GLYPH: Record<string, string> = {
+    k: '♚', q: '♛', r: '♜', b: '♝', n: '♞', p: '♟',
+  };
+
+  interface Cell { square: Square; piece: { glyph: string; color: string } | null; }
+
+  let cells = $derived.by<Cell[]>(() => {
+    const placement = fen.split(' ')[0];
+    const white = colors[BOARD_PLAYERS[id].w];
+    const black = colors[BOARD_PLAYERS[id].b];
+    const map: Record<string, { glyph: string; color: string }> = {};
+    placement.split('/').forEach((row, ri) => {
+      const rank = 8 - ri;
+      let file = 0;
+      for (const ch of row) {
+        if (/\d/.test(ch)) { file += Number(ch); continue; }
+        const isWhite = ch === ch.toUpperCase();
+        map[`${FILES[file]}${rank}`] = {
+          glyph: GLYPH[ch.toLowerCase()],
+          color: isWhite ? white : black,
+        };
+        file += 1;
+      }
+    });
+    const out: Cell[] = [];
+    for (let rank = 8; rank >= 1; rank--) {
+      for (let f = 0; f < 8; f++) {
+        const square = `${FILES[f]}${rank}`;
+        out.push({ square, piece: map[square] ?? null });
+      }
+    }
+    return out;
+  });
+
+  function classes(cell: Cell, index: number): string {
+    const dark = (index + Math.floor(index / 8)) % 2 === 1;
+    const hl = highlight;
+    const list = ['sq', dark ? 'dark' : 'light'];
+    if (ghosts.includes(cell.square)) list.push('ghost-sq');
+    if (hl?.playable.includes(cell.square)) list.push(cell.piece ? 'play occ' : 'play');
+    if (hl?.local.includes(cell.square)) list.push('local');
+    if (hl?.selected === cell.square) list.push('selected');
+    return list.join(' ');
+  }
+</script>
+
+<div class="board" class:active style="--rot:{BOARD_ROTATION[id]}deg">
+  {#each cells as cell, i (cell.square)}
+    <button class={classes(cell, i)} onclick={() => onSquare(cell.square)} aria-label={cell.square}>
+      {#if cell.piece}
+        <span class="pc" class:ghost={ghosts.includes(cell.square)}
+              style="color:{cell.piece.color}">{cell.piece.glyph}</span>
+      {/if}
+    </button>
+  {/each}
+</div>
+
+<style>
+  .board {
+    display: grid;
+    grid-template-columns: repeat(8, var(--sq, 34px));
+    grid-template-rows: repeat(8, var(--sq, 34px));
+    transform: rotate(var(--rot));
+    border: 1px solid #20232b;
+    border-radius: 3px;
+  }
+  .board.active { box-shadow: 0 0 0 3px var(--glow, #4a90d9), 0 0 20px 2px var(--glow, #4a90d9); }
+  .sq {
+    position: relative; padding: 0; border: 0; cursor: pointer;
+    display: flex; align-items: center; justify-content: center;
+  }
+  .sq.light { background: #cabf9f; }
+  .sq.dark { background: #7d6f55; }
+  .pc {
+    font-size: calc(var(--sq, 34px) * 0.76); line-height: 1;
+    text-shadow: -1px -1px 0 #15171c, 1px -1px 0 #15171c,
+                 -1px 1px 0 #15171c, 1px 1px 0 #15171c;
+  }
+  .pc.ghost { opacity: 0.42; }
+  .ghost-sq { outline: 2px dashed #888; outline-offset: -2px; }
+  .sq.play::after, .sq.local::after {
+    content: ''; position: absolute; border-radius: 50%;
+    width: 32%; height: 32%;
+  }
+  .sq.play::after { background: #46c24f; box-shadow: 0 0 7px #46c24f; }
+  .sq.play.occ::after {
+    width: 84%; height: 84%; background: transparent;
+    border: 3px solid #46c24f; box-shadow: 0 0 7px #46c24f;
+  }
+  .sq.local::after { background: transparent; border: 2px dashed #9aa0aa; }
+  .sq.selected { outline: 3px solid #3fd9d9; outline-offset: -3px; }
+</style>

src/lib/Compass.svelte

@@ -0,0 +1,83 @@
+<script lang="ts">
+  import Board from './Board.svelte';
+  import { gameStore } from './stores/game.svelte';
+  import { PLAYER_BOARDS } from '../engine/boards';
+  import type { BoardId, Player, Square } from '../engine/types';
+
+  const COLORS: Record<Player, string> = {
+    N: '#4a90d9', S: '#d6483f', E: '#9d5bd2', W: '#e08a3a',
+  };
+  // Board centre positions inside the 744x744 compass (see spec §5.1).
+  const POS: Record<BoardId, { left: number; top: number }> = {
+    NW: { left: 200, top: 200 }, NE: { left: 544, top: 200 },
+    SW: { left: 200, top: 544 }, SE: { left: 544, top: 544 },
+  };
+  const BOARD_IDS: BoardId[] = ['NW', 'NE', 'SW', 'SE'];
+
+  let view = $derived(gameStore.view);
+  let active = $derived(gameStore.activeBoards);
+
+  /** Ghost squares for a given board. */
+  function ghostsFor(id: BoardId): Square[] {
+    return view.ghosts.filter((g) => g.board === id).map((g) => g.square);
+  }
+
+  /** Highlight payload for a given board, or null. */
+  function highlightFor(id: BoardId) {
+    const h = gameStore.highlight;
+    if (h === null) return null;
+    if (id !== h.boardA && id !== h.boardB) return null;
+    const local = id === h.boardA ? h.onlyA : h.onlyB;
+    const selectedHere = active.includes(id) ? gameStore.selected : null;
+    return { playable: h.playable, local, selected: selectedHere };
+  }
+
+  function handleSquare(id: BoardId, square: Square): void {
+    if (gameStore.isScrubbing) return;
+    if (!active.includes(id)) return; // only the player-to-move's boards are interactive
+    if (gameStore.selected === null) {
+      gameStore.select(square);
+    } else if (gameStore.highlight?.playable.includes(square)) {
+      gameStore.commitTo(square);
+    } else {
+      gameStore.select(square); // re-grab or cancel
+    }
+  }
+</script>
+
+<div class="compass" style="--glow:{COLORS[view.currentPlayer]}">
+  {#each BOARD_IDS as id (id)}
+    <div class="slot" style="left:{POS[id].left}px; top:{POS[id].top}px;">
+      <Board
+        {id}
+        fen={view.fen[id]}
+        colors={COLORS}
+        ghosts={ghostsFor(id)}
+        highlight={highlightFor(id)}
+        active={active.includes(id)}
+        onSquare={(sq) => handleSquare(id, sq)}
+      />
+    </div>
+  {/each}
+
+  <div class="plabel" class:on={view.currentPlayer === 'N'}
+       style="left:372px; top:74px; background:{COLORS.N}">NORTH</div>
+  <div class="plabel" class:on={view.currentPlayer === 'S'}
+       style="left:372px; top:670px; background:{COLORS.S}">SOUTH</div>
+  <div class="plabel vert" class:on={view.currentPlayer === 'E'}
+       style="left:670px; top:372px; background:{COLORS.E}">EAST</div>
+  <div class="plabel vert" class:on={view.currentPlayer === 'W'}
+       style="left:74px; top:372px; background:{COLORS.W}">WEST</div>
+</div>
+
+<style>
+  .compass { position: relative; width: 744px; height: 744px; flex: none; }
+  .slot { position: absolute; transform: translate(-50%, -50%); }
+  .plabel {
+    position: absolute; transform: translate(-50%, -50%);
+    color: #fff; font-size: 12px; font-weight: 700; letter-spacing: 0.09em;
+    padding: 6px 12px; border-radius: 7px; white-space: nowrap;
+  }
+  .plabel.vert { writing-mode: vertical-rl; }
+  .plabel.on { box-shadow: 0 0 14px currentColor; }
+</style>

src/lib/Panel.svelte

@@ -0,0 +1,122 @@
+<script lang="ts">
+  import { gameStore } from './stores/game.svelte';
+  import { toCoordinate } from '../engine/notation';
+  import { PLAYERS } from '../engine/boards';
+  import type { Player } from '../engine/types';
+
+  const COLORS: Record<Player, string> = {
+    N: '#4a90d9', S: '#d6483f', E: '#9d5bd2', W: '#e08a3a',
+  };
+  const NAME: Record<Player, string> = { N: 'North', S: 'South', E: 'East', W: 'West' };
+
+  let view = $derived(gameStore.view);
+
+  /** Move log grouped into rounds of four (N,S,E,W). */
+  let rounds = $derived.by(() => {
+    const out: string[][] = [];
+    view.history.forEach((entry, i) => {
+      const r = Math.floor(i / 4);
+      (out[r] ??= [])[i % 4] = toCoordinate(entry);
+    });
+    return out;
+  });
+
+  let statusText = $derived.by(() => {
+    const s = view.status;
+    if (s.state === 'playing') return `${NAME[view.currentPlayer]} to move`;
+    if (s.state === 'checkmate') {
+      const winners = PLAYERS.filter((p) => s.result?.[p] === 'win').map((p) => NAME[p]);
+      return `Checkmate — ${NAME[view.currentPlayer]} loses; ${winners.join(' & ')} win`;
+    }
+    if (s.state === 'stalemate') return 'Stalemate — all draw';
+    return `Draw (${s.reason})`;
+  });
+
+  let fileInput: HTMLInputElement;
+
+  function onFile(e: Event): void {
+    const file = (e.target as HTMLInputElement).files?.[0];
+    if (file) gameStore.load(file).catch((err) => alert(String(err)));
+  }
+</script>
+
+<aside class="panel">
+  <section class="card">
+    <div class="turn">
+      <span class="dot" style="background:{COLORS[view.currentPlayer]}"></span>
+      {statusText}
+    </div>
+    <div class="sub">Ghosts on board: {view.ghosts.length} · Checks: {view.status.checks.length}</div>
+  </section>
+
+  <section class="card">
+    <h2>Move log</h2>
+    <table>
+      <thead><tr>
+        {#each PLAYERS as p}<th style="color:{COLORS[p]}">{NAME[p]}</th>{/each}
+      </tr></thead>
+      <tbody>
+        {#each rounds as round, r (r)}
+          <tr>
+            {#each [0, 1, 2, 3] as c}<td>{round[c] ?? ''}</td>{/each}
+          </tr>
+        {/each}
+      </tbody>
+    </table>
+  </section>
+
+  <section class="card">
+    <h2>Legend</h2>
+    <div class="legend">
+      <div><span class="ring play"></span> Playable — legal on both boards</div>
+      <div><span class="ring local"></span> Legal on that board only</div>
+      <div><span class="ring ghost"></span> Ghost — twin captured, frozen</div>
+    </div>
+  </section>
+
+  <section class="card controls">
+    <button onclick={() => gameStore.newGame()}>New game</button>
+    <button onclick={() => gameStore.undo()} disabled={view.ply === 0}>Undo</button>
+    <button onclick={() => gameStore.scrubTo((gameStore.scrubPly ?? view.ply) - 1)}
+            disabled={view.ply === 0}>◀ Prev</button>
+    <button onclick={() => gameStore.scrubTo((gameStore.scrubPly ?? view.ply) + 1)}
+            disabled={!gameStore.isScrubbing}>Next ▶</button>
+    <button onclick={() => gameStore.scrubTo(null)} disabled={!gameStore.isScrubbing}>● Live</button>
+    <button onclick={() => gameStore.declareDraw()}>Declare draw</button>
+    <button onclick={() => gameStore.save()}>Save</button>
+    <button onclick={() => fileInput.click()}>Load</button>
+    <input type="file" accept="application/json" bind:this={fileInput}
+           onchange={onFile} style="display:none" />
+  </section>
+</aside>
+
+<style>
+  .panel { width: 290px; display: flex; flex-direction: column; gap: 13px; }
+  .card {
+    background: #1d2027; border: 1px solid #333845;
+    border-radius: 9px; padding: 13px 15px;
+  }
+  .card h2 {
+    margin: 0 0 8px; font-size: 11px; letter-spacing: 0.07em;
+    text-transform: uppercase; color: #9aa0aa;
+  }
+  .turn { display: flex; align-items: center; gap: 9px; font-weight: 600; }
+  .dot { width: 13px; height: 13px; border-radius: 50%; }
+  .sub { font-size: 12px; color: #9aa0aa; margin-top: 6px; }
+  table { width: 100%; border-collapse: collapse; font-size: 12px; }
+  th, td { padding: 3px 5px; text-align: left; }
+  th { font-size: 10px; text-transform: uppercase; }
+  td { font-family: ui-monospace, Menlo, monospace; color: #cdd2da; }
+  .legend { display: flex; flex-direction: column; gap: 7px; font-size: 12px; }
+  .legend div { display: flex; align-items: center; gap: 8px; }
+  .ring { width: 14px; height: 14px; border-radius: 50%; flex: none; }
+  .ring.play { background: #46c24f; }
+  .ring.local { border: 2px dashed #9aa0aa; }
+  .ring.ghost { border: 2px dashed #888; }
+  .controls { display: flex; flex-wrap: wrap; gap: 6px; }
+  .controls button {
+    background: #262b34; color: #e6e8ec; border: 1px solid #333845;
+    border-radius: 5px; padding: 5px 9px; font-size: 12px; cursor: pointer;
+  }
+  .controls button:disabled { opacity: 0.4; cursor: default; }
+</style>

src/lib/PromotionDialog.svelte

@@ -0,0 +1,44 @@
+<script lang="ts">
+  import { gameStore } from './stores/game.svelte';
+  import type { PromotionPiece } from '../engine/types';
+
+  const PIECES: { code: PromotionPiece; glyph: string }[] = [
+    { code: 'q', glyph: '♛' }, { code: 'r', glyph: '♜' },
+    { code: 'b', glyph: '♝' }, { code: 'n', glyph: '♞' },
+  ];
+
+  let pending = $derived(gameStore.pendingPromotion);
+</script>
+
+{#if pending}
+  <div class="backdrop" onclick={() => gameStore.cancelPromotion()}
+       role="presentation">
+    <div class="dialog" onclick={(e) => e.stopPropagation()} role="presentation">
+      <h3>Promote pawn ({pending.from}→{pending.to})</h3>
+      <div class="row">
+        {#each PIECES as p}
+          <button onclick={() => gameStore.choosePromotion(p.code)}>{p.glyph}</button>
+        {/each}
+      </div>
+    </div>
+  </div>
+{/if}
+
+<style>
+  .backdrop {
+    position: fixed; inset: 0; background: rgba(0, 0, 0, 0.6);
+    display: flex; align-items: center; justify-content: center; z-index: 50;
+  }
+  .dialog {
+    background: #1d2027; border: 1px solid #333845;
+    border-radius: 10px; padding: 18px 22px;
+  }
+  .dialog h3 { margin: 0 0 12px; font-size: 14px; }
+  .row { display: flex; gap: 10px; }
+  .row button {
+    font-size: 38px; line-height: 1; width: 60px; height: 60px;
+    background: #262b34; color: #e6e8ec;
+    border: 1px solid #333845; border-radius: 8px; cursor: pointer;
+  }
+  .row button:hover { border-color: #46c24f; }
+</style>

src/lib/stores/game.svelte.ts

@@ -0,0 +1,164 @@
+import { DuplicateGame } from '../../engine/game';
+import { legalSyncedMoves, selectionHighlight, type SelectionHighlight } from '../../engine/legality';
+import { ghosts } from '../../engine/ghosts';
+import { evaluateStatus } from '../../engine/endgame';
+import { serialize, deserialize } from '../../engine/notation';
+import { PLAYER_BOARDS } from '../../engine/boards';
+import type {
+  BoardId, Player, Square, SyncMove, HistoryEntry, GhostMarker, GameStatus,
+} from '../../engine/types';
+
+/** A plain, reactivity-friendly snapshot of everything the UI renders. */
+export interface GameView {
+  /** Piece-placement FEN field per board. */
+  fen: Record<BoardId, string>;
+  currentPlayer: Player;
+  ply: number;
+  ghosts: GhostMarker[];
+  status: GameStatus;
+  history: HistoryEntry[];
+}
+
+function buildView(game: DuplicateGame): GameView {
+  return {
+    fen: {
+      NW: game.boards.NW.fen(),
+      NE: game.boards.NE.fen(),
+      SW: game.boards.SW.fen(),
+      SE: game.boards.SE.fen(),
+    },
+    currentPlayer: game.currentPlayer,
+    ply: game.ply,
+    ghosts: ghosts(game),
+    status: evaluateStatus(game),
+    history: [...game.history],
+  };
+}
+
+class GameStore {
+  /** The authoritative live game — deliberately NOT a $state proxy. */
+  #game = new DuplicateGame();
+
+  /** Snapshot the UI renders. While scrubbing it reflects a past ply. */
+  view = $state<GameView>(buildView(this.#game));
+  /** The grabbed square, or null. */
+  selected = $state<Square | null>(null);
+  /** Triple-highlight for the grabbed piece, or null. */
+  highlight = $state<SelectionHighlight | null>(null);
+  /** A pawn move awaiting a promotion choice, or null. */
+  pendingPromotion = $state<{ from: Square; to: Square } | null>(null);
+  /** Ply currently being viewed; null means the live position. */
+  scrubPly = $state<number | null>(null);
+
+  get isScrubbing(): boolean {
+    return this.scrubPly !== null;
+  }
+
+  /** Which boards belong to the player to move (for the turn glow). */
+  get activeBoards(): [BoardId, BoardId] {
+    return PLAYER_BOARDS[this.#game.currentPlayer];
+  }
+
+  /** Grab a piece: must be the current player's turn and a live (non-scrub) view. */
+  select(square: Square): void {
+    if (this.isScrubbing) return;
+    if (this.selected === square) { this.clearSelection(); return; }
+    this.selected = square;
+    this.highlight = selectionHighlight(this.#game, square);
+  }
+
+  clearSelection(): void {
+    this.selected = null;
+    this.highlight = null;
+  }
+
+  /** Attempt to play the grabbed piece to `to`. Opens the promotion dialog if needed. */
+  commitTo(to: Square): void {
+    const from = this.selected;
+    if (from === null || this.highlight === null) return;
+    if (!this.highlight.playable.includes(to)) return; // not a synchronized-legal square
+
+    const moves = legalSyncedMoves(this.#game).filter((m) => m.from === from && m.to === to);
+    if (moves.length === 0) return;
+    if (moves.some((m) => m.promotion)) {
+      this.pendingPromotion = { from, to };
+      return;
+    }
+    this.#apply(moves[0]);
+  }
+
+  /** Finish a promotion started by commitTo. */
+  choosePromotion(piece: SyncMove['promotion']): void {
+    if (this.pendingPromotion === null) return;
+    this.#apply({ ...this.pendingPromotion, promotion: piece });
+    this.pendingPromotion = null;
+  }
+
+  cancelPromotion(): void {
+    this.pendingPromotion = null;
+  }
+
+  #apply(move: SyncMove): void {
+    this.#game.applyMove(move);
+    this.clearSelection();
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  undo(): void {
+    this.#game.undo();
+    this.clearSelection();
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  newGame(): void {
+    this.#game = new DuplicateGame();
+    this.clearSelection();
+    this.pendingPromotion = null;
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+
+  /** Scrub the move history; null returns to the live position. */
+  scrubTo(ply: number | null): void {
+    this.clearSelection();
+    if (ply === null || ply >= this.#game.ply) {
+      this.scrubPly = null;
+      this.view = buildView(this.#game);
+      return;
+    }
+    this.scrubPly = ply;
+    this.view = buildView(new DuplicateGame(this.#game.history.slice(0, ply)));
+  }
+
+  /** Manually declare a draw (provisional: insufficient material is not auto-detected). */
+  declareDraw(): void {
+    this.view = {
+      ...this.view,
+      status: { state: 'draw', reason: 'manual', checks: [],
+        result: { N: 'draw', S: 'draw', E: 'draw', W: 'draw' } },
+    };
+  }
+
+  save(): void {
+    const blob = new Blob([serialize(this.#game.history)], { type: 'application/json' });
+    const url = URL.createObjectURL(blob);
+    const a = document.createElement('a');
+    a.href = url;
+    a.download = `duplicate-chess-${Date.now()}.json`;
+    a.click();
+    URL.revokeObjectURL(url);
+  }
+
+  async load(file: File): Promise<void> {
+    const history = deserialize(await file.text());
+    this.#game = new DuplicateGame(history);
+    this.clearSelection();
+    this.pendingPromotion = null;
+    this.scrubPly = null;
+    this.view = buildView(this.#game);
+  }
+}
+
+export const gameStore = new GameStore();

src/main.ts

@@ -0,0 +1,9 @@
+import { mount } from 'svelte'
+import './app.css'
+import App from './App.svelte'
+
+const app = mount(App, {
+  target: document.getElementById('app')!,
+})
+
+export default app

svelte.config.js

@@ -0,0 +1,2 @@
+/** @type {import("@sveltejs/vite-plugin-svelte").SvelteConfig} */
+export default {}

tsconfig.app.json

@@ -0,0 +1,20 @@
+{
+  "extends": "@tsconfig/svelte/tsconfig.json",
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
+    "target": "es2023",
+    "module": "esnext",
+    "types": ["svelte", "vite/client"],
+    "noEmit": true,
+    /**
+     * Typecheck JS in `.svelte` and `.js` files by default.
+     * Disable checkJs if you'd like to use dynamic types in JS.
+     * Note that setting allowJs false does not prevent the use
+     * of JS in `.svelte` files.
+     */
+    "allowJs": true,
+    "checkJs": true,
+    "moduleDetection": "force"
+  },
+  "include": ["src/**/*.ts", "src/**/*.js", "src/**/*.svelte"]
+}

tsconfig.json

@@ -0,0 +1,7 @@
+{
+  "files": [],
+  "references": [
+    { "path": "./tsconfig.app.json" },
+    { "path": "./tsconfig.node.json" }
+  ]
+}

tsconfig.node.json

@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.node.tsbuildinfo",
+    "target": "es2023",
+    "lib": ["ES2023"],
+    "module": "esnext",
+    "types": ["node"],
+    "skipLibCheck": true,
+
+    /* Bundler mode */
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+
+    /* Linting */
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true
+  },
+  "include": ["vite.config.ts"]
+}

vite.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vite'
+import { svelte } from '@sveltejs/vite-plugin-svelte'
+
+// https://vite.dev/config/
+export default defineConfig({
+  plugins: [svelte()],
+})

vitest.config.ts

@@ -0,0 +1,5 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: { environment: 'node', include: ['src/**/*.test.ts'] },
+});