Implements backend multiplexed WebSocket server on /mux endpoint:
- Add mux-protocol.ts with ClientMessage and ServerMessage types
- Create TerminalManager class for managing PTY processes independently
- Implement mux-websocket.ts with attachMuxWebSocket() function
- Wire mux server into existing direct-terminal-ws server
- Support multiple terminals over single persistent WebSocket connection
- Implement 50KB ring buffer for background terminal output
- Add heartbeat and reconnection logic
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Gaurav Bhola