init: scaffold duplicate_chess project and design spec

Local browser sandbox for "duplicate chess" — a four-player coupled-board
chess variant invented by Andrew Freiberg. Scaffold per CREATE_PROJECT.md
plus the approved design spec from this session's brainstorming.

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

IDEA.md

@@ -0,0 +1,42 @@
+# IDEA.md — Duplicate Chess
+
+## What is this?
+
+A local, browser-based sandbox/visualizer for **"duplicate chess"** — a four-player
+chess variant invented by Andrew Freiberg (Seth's father). This project is to
+duplicate chess what the sibling `blind_chess` project is to blind chess: a
+digitization that lets an otherwise hard-to-play variant actually be played and
+studied.
+
+Duplicate chess: four players — North, South, East, West — and four chessboards set
+up between adjacent compass points (NW, NE, SW, SE). North and South play White;
+East and West play Black. Each player controls one colour on **two** boards (North =
+White on NW and NE; West = Black on NW and SW; and so on). Turn order is N → S → E → W.
+
+The defining rule: on your turn you make the **identical algebraic move on both of
+your boards**. A move is legal only if it is legal on both. When one of your pieces
+is captured on one board, its twin on your other board freezes into a **"ghost"** —
+it can never move again (no synchronized move exists for it), but it still occupies
+its square, blocks lines, can give discovered check, and can itself be captured.
+Every individual board always remains a legal game of orthodox chess.
+
+## Problem it solves
+
+In the inventor's own words, the variant can't be grasped from prose: "this is not
+really a variant that can be understood verbally after a certain point ... diagrams
+are probably essential." The physical game is fiddly to set up and play. This tool
+puts all four boards on one screen, enforces the synchronized-move coupling, renders
+ghosts, and shows *why* a move is or isn't legal — so the variant can be seen.
+
+## Constraints / preferences
+
+- Local, single-operator tool. No server, no networking, no accounts — duplicate
+  chess is perfect-information, so one screen shows the whole game truthfully.
+- Stack: Vite + Svelte 5 + TypeScript, `chess.js` per board. Single package.
+- Sibling project to `blind_chess`; same homelab conventions.
+- v1 is a visualizer/sandbox. Networked four-player play and AI opponents are
+  explicitly out of scope for now.
+
+See `docs/superpowers/specs/2026-05-19-duplicate-chess-design.md` for the full design,
+and `USERFILES/4-person-chess.txt` in the `blind_chess` repo for the original
+inventor conversation this is based on.