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

ts/proxybridge.ts

@@ -238,6 +238,38 @@ export async function webrtcLink(sessionId: string, callId: string, providerMedi
   }
 }
 
+/**
+ * Add an external SIP leg to an existing call (multiparty).
+ */
+export async function addLeg(callId: string, number: string, providerId?: string): Promise<string | null> {
+  if (!bridge || !initialized) return null;
+  try {
+    const result = await bridge.sendCommand('add_leg', {
+      call_id: callId,
+      number,
+      provider_id: providerId,
+    } as any);
+    return (result as any)?.leg_id || null;
+  } catch (e: any) {
+    logFn?.(`[proxy-engine] add_leg error: ${e?.message || e}`);
+    return null;
+  }
+}
+
+/**
+ * Remove a leg from a call.
+ */
+export async function removeLeg(callId: string, legId: string): Promise<boolean> {
+  if (!bridge || !initialized) return false;
+  try {
+    await bridge.sendCommand('remove_leg', { call_id: callId, leg_id: legId } as any);
+    return true;
+  } catch (e: any) {
+    logFn?.(`[proxy-engine] remove_leg error: ${e?.message || e}`);
+    return false;
+  }
+}
+
 /**
  * Close a WebRTC session.
  */