Hot restart frees the old isolate's NativeCallables while mpv handles stay
alive; media_kit then quits them on the next start and mpv calls the dead
wakeup callback (SIGSEGV on the mpv 'core' thread). A debug-only runner
hook clears every live handle's wakeup callback on the engine's
on-pre-engine-restart signal.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>