Fix WebRTC signaling state error in pooled services

- Add signaling state validation before setting remote answer
- Fix race condition by removing offers from pool before processing
- Add detailed debug logging for state mismatch errors
- Prevent duplicate processing of answered offers

This fixes the "Cannot set remote answer in state stable" error
that occurred when multiple answers arrived in quick succession.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/rondevu.ts

@@ -177,6 +177,7 @@ export class Rondevu {
     return new RondevuPeer(
       this._offers,
       rtcConfig,
+      undefined, // No existing peer connection
       this.rtcPeerConnection,
       this.rtcSessionDescription,
       this.rtcIceCandidate