Addresses the first-join no-audio regression (tasks #35-37) where
the Oboe playout callback fires once (cb#0) and then stops
draining the ring on the Nothing Phone, causing written_samples
to freeze at 7679 (ring capacity minus one burst). Second call
(rejoin) always works because audio_stop tears down the streams
and audio_start rebuilds them fresh.
Two combined fixes:
**Fix D (task #37)**: always call audio_stop() before audio_start()
at the top of CallEngine::start. On a cold launch this is a no-op
(streams not yet started). On subsequent calls it guarantees a
clean teardown before rebuild — the same thing rejoin does. Added
a 50ms pause between stop and start to let the Android HAL release
the audio session.
**Fix C (task #36)**: after audio_start(), immediately write 960
samples (20ms) of silence into the playout ring. This ensures the
Oboe playout callback has data to drain on its first invocation.
On devices where an empty-ring first callback causes the stream
to self-pause (Nothing Phone's Qualcomm HAL), the priming data
keeps the callback loop alive until real decoded audio arrives
from the recv task.
Together these cover the two most likely root causes:
1. Stale Oboe state from a previous audio_start that didn't
clean up properly → Fix D forces a clean rebuild
2. Playout callback self-pausing on an empty ring → Fix C
ensures the ring is non-empty at callback time
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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