feat(bot): vanilla CasualBrain delegates to js-chess-engine

The hand-rolled scoring heuristic lost to a random-move baseline 7-7 in
self-play — far below the spec's >=80% acceptance bar. Swap in a real
chess engine (js-chess-engine, MIT, ~400KB, no native deps) for vanilla
mode at level 2 with randomness=30 to break threefold cycles.

- BrainInput.fen added; driver populates it ONLY in vanilla mode.
  Blind mode omits the FEN so the engine path can't smuggle opponent
  positions past the view filter.
- CasualBrain in vanilla: convert FEN -> EngineGame -> ai({level: 2});
  validate the engine's move is in legalCandidates; fall back to
  heuristic on miss.
- Blind mode unchanged (engine isn't useful when only own pieces are
  visible — that's Phase 2 Recon's territory).

Self-play vs RandomBrain (100 games each direction, vanilla):
  - Casual(W) vs Random(B): W=97%
  - Random(W) vs Casual(B): B=96%
Casual-vs-Casual vanilla balanced, ~5-30ms/move. All 54 tests still pass.

Refresh .secrets.baseline (stale) to allow new pnpm-lock.yaml hashes.

packages/server/package.json

@@ -17,6 +17,7 @@
     "@fastify/websocket": "^11.0.0",
     "chess.js": "^1.4.0",
     "fastify": "^5.2.0",
+    "js-chess-engine": "^2.4.6",
     "pino": "^9.5.0",
     "ws": "^8.18.0",
     "zod": "^3.24.0"