Initializes prefetch on external subtitle track activation, detects
seeks via time-pos delta threshold, pauses prefetch during live
subtitle processing, and restarts on cache invalidation.
- Extract loadSubtitleSourceText into reusable function
- Add prefetch service state and initSubtitlePrefetch helper
- Thread onTimePosUpdate through event actions/bindings/main-deps
- Pause prefetch on subtitle change, resume on emit
- Restart prefetch after tokenization cache invalidation
- Query track-list on media path change to find external subs