Streams text to the ElevenLabs WebSocket API and plays audio in real-time
via AudioTrack (PCM 24kHz). Key design points:
- sendText(fullText) takes the full accumulated text and only transmits the
new suffix, detecting divergence for restart
- Chunks are queued if the WebSocket isn't yet connected; flushed in onOpen
- finish() sends EOS to ElevenLabs; deferred if called before onOpen fires
- sendText returns true (not false) when finished=true to avoid treating a
normal end-of-stream as a diverge restart
- finishStreamingTts coroutine uses identity check before nulling streamingTts
to prevent a mid-drain restart from orphaning a live TTS session
- eleven_v3 does NOT support WebSocket streaming; use eleven_flash_v2_5
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