Implement a real-time game server using protobuf over WebSocket:
- Protobuf schema with 14 message types in oneof Envelope
- Room-based lobby with join/leave/chat broadcast
- ELO-based matchmaking with expanding range (±100 to ±500)
- Turn-based game session lifecycle management
- Multi-stage Docker build (Alpine)
- Unit tests for matchmaking and game session logic
