SIGSEGV on hangup: capture thread calls writeAudio() via JNI after
teardown() has freed the native engine handle. TOCTOU race between
the nativeHandle==0L check and destroy() on the ViewModel thread.
Fix: CountDownLatch(2) — audio threads count down after exiting loops,
teardown() awaits before destroy(). 2 Kotlin files, no Rust changes.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
