fix(voice-call): prevent audio overlap with TTS queue (#1713)

* fix(voice-call): prevent audio overlap with TTS queue

Add a TTS queue to serialize audio playback and prevent overlapping
speech during voice calls. Previously, concurrent speak() calls could
send audio chunks simultaneously, causing garbled/choppy output.

Changes:
- Add queueTts() to MediaStreamHandler for sequential TTS playback
- Wrap playTtsViaStream() audio sending in the queue
- Clear queue on barge-in (when user starts speaking)

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

* fix(voice-call): use iterative queue processing to prevent heap exhaustion

The recursive processQueue() pattern accumulated stack frames, causing
JavaScript heap out of memory errors on macOS CI. Convert to while loop
for constant stack usage regardless of queue depth.

* fix: prevent voice-call TTS overlap (#1713) (thanks @dguido)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

extensions/voice-call/src/webhook.ts

@@ -78,6 +78,11 @@ export class VoiceCallWebhookServer {
           `[voice-call] Transcript for ${providerCallId}: ${transcript}`,
         );
 
+        // Clear TTS queue on barge-in (user started speaking, interrupt current playback)
+        if (this.provider.name === "twilio") {
+          (this.provider as TwilioProvider).clearTtsQueue(providerCallId);
+        }
+
         // Look up our internal call ID from the provider call ID
         const call = this.manager.getCallByProviderCallId(providerCallId);
         if (!call) {
@@ -109,6 +114,11 @@ export class VoiceCallWebhookServer {
           });
         }
       },
+      onSpeechStart: (providerCallId) => {
+        if (this.provider.name === "twilio") {
+          (this.provider as TwilioProvider).clearTtsQueue(providerCallId);
+        }
+      },
       onPartialTranscript: (callId, partial) => {
         console.log(`[voice-call] Partial for ${callId}: ${partial}`);
       },