Enhance the WASM game sandbox with production-accurate Go engine:
- Add multi-player support (2-4 players) to Go WASM engine via JS callbacks
- New acbEngine.addPlayer/clearPlayers/runMatchMulti API for N-player matches
- Sandbox auto-loads Go WASM engine in background, falls back to TS engine
- Engine selector: Auto (Go WASM → TS fallback), Go WASM only, or TS only
- Engine status indicator shows which engine is active
- Performance panel reports which engine was used
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>