Function audio_engine_eof_watchdog_start 

pub fn audio_engine_eof_watchdog_start(app: AppHandle)

Start a background thread that polls playback_status every EOF_WATCHDOG_POLL_MS and emits audio-engine-playback-eof when loaded && eof transitions to true (same edge audio-engine.js uses for autoplay next). Idempotent — if already running, returns immediately.