H7: Process 'exit'/'SIGINT' listeners registered per-session were
never removed when all sessions disconnected, accumulating handlers.
- Add unregisterProcessCleanup() called in disconnectAll()
H8: Race condition where disconnectSession() during pending connection
left orphan clients in state.clients.
- Add disconnectedSessions Set to track mid-flight disconnects
- Check disconnect marker after connection resolves, close if stale
- Clear marker on reconnection for same session
Tests: 6 pass (3 disconnect + 3 race)
YeonGyu-Kim