TTS: chunked utterances; buffer-then-play allowed as a first version

Brings the audio sink notes in line with project-yukart decision 12 and
ORIN-RUNTIME: the wire format is chunks plus terminator from the start,
and the first receiver may buffer to the terminator, so moving to
streaming playback is a receiver-only change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/impl/plans/INITIAL.md

@@ -84,8 +84,13 @@ itself, not in a settings page or a startup log.
 The tablet owns the car's aux input and is the music player, so the Orin's speech
 has to arrive here to be heard.
 
-- TTS arrives as **discrete utterances** over the reliable channel, not as a
-  real-time stream. Receive the whole thing, then play it — no jitter buffer.
+- TTS arrives as an **utterance: a sequence of chunks with a terminator** over the
+  reliable channel, never as one opaque file body. Streaming playback (start on
+  the first chunk, append, finish on the terminator via `AudioTrack` streaming
+  mode) is the target.
+- **Acceptable first version:** buffer the chunks and play on the terminator. With
+  a non-streaming TTS that is a single chunk anyway. Swapping to streaming then
+  changes only this receiver, not the protocol or the Orin.
 - Duck other audio with `AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK` for the duration of
   the utterance, then release. That is what makes Spotify drop under the voice
   without either side coordinating.