Connection Lifecycle

1. WebSocket connect
2. Server → ready { id, protocolVersion }
3. Client → offer (SDP)
4. Server → signal (SDP answer)
5. WebRTC data channel established
6. Heartbeats: ping / pong
7. Reconnect: client sends reconnect with previous sessionId; server replies reconnect-ack

Payload messages (transcripts, llm, tts, etc.) are sent on both channels; client should process from data channel when available, fallback to WebSocket otherwise.