feat(engine): board/player constant maps and shared types

src/engine/boards.test.ts

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+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

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+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/types.ts

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+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[];
+}