One PCM16 audio chunk. Orchestrator forwards to the client.
Synthesis drained after flush() or cancel(). Emitted exactly once per
turn, and never after cancel() for the cancelled turn: cancel() must
clear any pending done timers/frames so a stale done cannot leak into
the next turn's flush-wait (the event carries no turn id, so the
pipeline transport cannot filter it — see pipeline-transport.ts).
Terminal error. The session is expected to end after this fires.
Events emitted by an open TtsSession.