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>

CLAUDE.md

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+# Duplicate Chess
+
+> Local browser sandbox for "duplicate chess" — a four-player coupled-board chess variant.
+
+## Start Here
+
+**Read the latest handoff first:** `.claude/handoffs/` (most recent file).
+It has session state, in-progress work, and ordered next steps.
+
+Then check `IDEA.md` for the project brief, `DECISIONS.md` for settled choices, and
+`docs/superpowers/specs/2026-05-19-duplicate-chess-design.md` for the full design.
+
+## Project Identity
+
+A digitization of "duplicate chess," a four-player chess variant invented by Andrew
+Freiberg (Seth's father — also the inventor of the blind-chess variant behind the
+sibling `blind_chess` project). 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 algebraic move on both. A captured piece leaves a frozen, capturable
+"ghost" twin on the player's other board. Every individual board always remains a
+legal game of orthodox chess.
+
+This project is a **local, single-operator sandbox/visualizer**: one screen, all four
+boards, an engine that enforces the synchronized-move coupling, renders ghosts, and
+detects the endgame. Its purpose is to let the variant be *seen* and studied — the
+inventor's own point is that it cannot be understood from prose alone.
+
+## Current State
+
+- **Phase:** spec complete; implementation not started.
+- **Repo:** `git.sethpc.xyz/Seth/duplicate_chess`.
+- **Deploy target:** n/a — runs locally (`pnpm dev`).
+- **Stack:** Vite + Svelte 5 + TypeScript, `chess.js`. Single package; engine in
+  `src/engine/` (pure, DOM-free, vitest-tested), UI in `src/lib/`.
+- **Next:** implementation plan via the writing-plans skill, then build.
+
+## Conventions
+
+- Inherits homelab conventions from `~/bin/CLAUDE.md` (gitea CLI, conventional
+  commits, `.claude/handoffs/` for session state).
+- The engine (`src/engine/`) is DOM-free and the single source of truth for game
+  state. The UI never computes legality itself.
+- Provisional endgame rules are marked PROVISIONAL in the spec — they are Claude's
+  chosen defaults, not Andrew's rulings. Flag them if revisited.