Fix WebRTC state machine error in pooled services

When handling answered offers in pooled services, we were creating fresh
peer connections in "stable" state and trying to set the remote answer,
which caused "Cannot set remote answer in state stable" error.

Fixed by:
- Adding offerSdp to AnsweredOffer interface
- Passing original offer SDP through the offer pool
- Setting local description (offer) before remote description (answer)

This ensures the peer connection is in "have-local-offer" state before
applying the answer, satisfying WebRTC's state machine requirements.

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

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

src/offer-pool.ts

@@ -6,7 +6,8 @@ import { RondevuOffers, Offer } from './offers.js';
 export interface AnsweredOffer {
   offerId: string;
   answererId: string;
-  sdp: string;
+  sdp: string; // Answer SDP
+  offerSdp: string; // Original offer SDP
   answeredAt: number;
 }
 
@@ -110,11 +111,18 @@ export class OfferPool {
 
       // Process each answer
       for (const answer of myAnswers) {
-        // Notify ServicePool
+        // Get the original offer
+        const offer = this.offers.get(answer.offerId);
+        if (!offer) {
+          continue; // Offer already consumed, skip
+        }
+
+        // Notify ServicePool with both answer and original offer SDP
         await this.options.onAnswered({
           offerId: answer.offerId,
           answererId: answer.answererId,
           sdp: answer.sdp,
+          offerSdp: offer.sdp,
           answeredAt: answer.answeredAt
         });
 

src/service-pool.ts

@@ -241,7 +241,13 @@ export class ServicePool {
       peer.role = 'offerer';
       peer.offerId = answer.offerId;
 
-      // Set remote description (the answer)
+      // Set local description (the original offer) first
+      await peer.pc.setLocalDescription({
+        type: 'offer',
+        sdp: answer.offerSdp
+      });
+
+      // Now set remote description (the answer)
       await peer.pc.setRemoteDescription({
         type: 'answer',
         sdp: answer.sdp