The track-building effect in VideoContext reruns once api?.basePath and
isCurrentSubImageBased settle. An earlier async run could resolve after a
rerun and overwrite subtitleTracks/audioTracks with setTrack callbacks bound
to a stale `api`, breaking external-subtitle identity matching.
Add a cancellation token and route every state commit through guarded
committers so all six commit points (offline-transcoded audio/subs,
burned-in, and the online audio/subs paths) drop writes from a dead run,
plus bail out right after the awaited getAudioTracks when cancelled.