Agent events arrive on multiple threads concurrently. A stale event
with shorter accumulated text was falsely triggering 'text diverged',
causing the streaming TTS to restart with a new WebSocket — resulting
in multiple simultaneous ElevenLabs connections (2-3 voices) and
eventual system TTS fallback when hasReceivedAudio was false.
Fix: if sentFullText.startsWith(fullText), the event is stale (we
already have this text), not diverged. Accept and ignore it.
ccca99c472