Add GameNamespaceHandler with full event handling for real-time gameplay:
- handle_join: Join/rejoin games with visibility-filtered state
- handle_action: Execute actions and broadcast state to participants
- handle_resign: Process resignation and end game
- handle_disconnect: Notify opponent of disconnection
- Broadcast helpers for state, game over, and opponent status
Includes 28 unit tests covering all handler methods.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
