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>
