Two races shared the same cause: pushSamples runs on the audio capture thread
while clear() runs on the Compose main thread.
1. Lost redraw notifications
Both paths did `_revision.value += 1`. That expands to get -> add -> set and is
not atomic. A controlled two-thread probe (20k increments each, five runs)
lost up to 6,402 increments / 16%; using StateFlow.update lost zero. Since
revision is the Canvas's only redraw signal, every lost update can leave the
waterfall showing stale rows. If both writes land on the same number, StateFlow
sees no value change and notifies nobody.
Use `_revision.update { it + 1 }` in both paths.
2. Clear resurrected pre-clear audio
pushSamples copies pending audio under the lock, deliberately performs FFT
outside it, then reacquires the lock to append rows. The exact interleaving:
audio thread: take old audio, start FFT
main thread: user taps Clear -> rows/pending empty
audio thread: old FFT completes -> appends old rows again
The display becomes empty then immediately redraws the audio the user cleared.
A deterministic thread probe reproduced old rows after clear. Add a generation
counter protected by the same lock: pushSamples records it before FFT and drops
the computed rows when clear incremented it meanwhile. Fixed probe remains empty.
Verification: :feature:cw:compileReleaseKotlin + full :core:domain:test BUILD
SUCCESSFUL; grep confirms no non-atomic revision increments remain.
mckero
19ca5205fb