Computer play: Built and tested Stockfish Socket.IO client

[Ultree1]

Summary

Resolves #9:
Build a tested Socket.IO client for communicating with the Stockfish chess engine server.

New required env var: add EXPO_PUBLIC_STOCKFISH_SOCKET_URL=http://10.0.2.2:8080 to your .env.local, or the app won't start.

New files:

• src/features/play/services/StockfishClient.ts - StockfishClient implementation, interfaces, and response validation.
• test/StockfishClient.test.ts - Tests for StockfishClient.
• test/mocks/FakeSocket.ts - Fake socket used by the tests in place of a real server connection.
• docs/stockfish-socket-contract.md - Documentation of the Stockfish server's socket events, payloads, and unresolved details.

Modified files:

• src/config/env.ts - Validates the new EXPO_PUBLIC_STOCKFISH_SOCKET_URL.
• .env.example, test/setup.ts, test/env.test.ts - Added the new env var and its tests.

Developers can create a StockfishClient via createStockfishClient(), optionally passing in a url (defaults to the env var), a createSocket factory, and timeoutMs.

Using this client, they can call startSession(), requestMove() with a FEN, player move, and difficulty level 1–5, and disconnect(). The client connects lazily, validates server replies with zod, and rejects on timeouts, connection failures, and server errors without leaving listeners behind. Screens never access the socket directly.

Notes:

• requestMove() returns only the engine's move. The server's newFEN is wrong whenever a player move is sent (details in the contract doc).
• Difficulty 1–5 is sent directly as engine search depth.
• Only one move request can run at a time, because server replies carry no request ID.
• Auto-reconnect is off. The server deletes the session on disconnect, so callers run startSession() again.
• Auth and rate limits don't exist on the server. Documented as unresolved.

Tests

Run the new tests with: npm run test -- ./test/StockfishClient.test.ts --verbose.
This should be the result: Tests: 33 passed, 33 total