feat(proxy-engine): add multiparty call mixing with dynamic SIP and WebRTC leg management

ts/sipproxy.ts

@@ -39,6 +39,8 @@ import {
   webrtcIce,
   webrtcLink,
   webrtcClose,
+  addLeg,
+  removeLeg,
 } from './proxybridge.ts';
 import type {
   IIncomingCallEvent,
@@ -359,6 +361,19 @@ async function startProxyEngine(): Promise<void> {
     log(`[sip] unhandled ${data.method_or_status} Call-ID=${data.call_id?.slice(0, 20)} from=${data.from_addr}:${data.from_port}`);
   });
 
+  // Leg events (multiparty).
+  onProxyEvent('leg_added', (data: any) => {
+    log(`[leg] added: call=${data.call_id} leg=${data.leg_id} kind=${data.kind} state=${data.state}`);
+  });
+
+  onProxyEvent('leg_removed', (data: any) => {
+    log(`[leg] removed: call=${data.call_id} leg=${data.leg_id}`);
+  });
+
+  onProxyEvent('leg_state_changed', (data: any) => {
+    log(`[leg] state: call=${data.call_id} leg=${data.leg_id} → ${data.state}`);
+  });
+
   // WebRTC events from Rust — forward ICE candidates to browser via WebSocket.
   onProxyEvent('webrtc_ice_candidate', (data: any) => {
     // Find the browser's WebSocket by session ID and send the ICE candidate.