Destroy the mpv instance before the track-change router.replace in
replaceWithTrackSelection and goToPreviousItem, matching goToNextItem — two
overlapping decoders/surfaces during the transition OOM low-RAM devices
(CodeRabbit). addSubtitleFile now sets the live index to the downloaded local
sub's real index (LOCAL_SUBTITLE_INDEX_START - position) instead of a blanket
-100, which collided with the first local sub and mis-recorded the selection
(Copilot); it still resolves to notFound on the onTracksReady re-apply so it
doesn't clobber the freshly selected track.