Subtitle downloads were going to a shared /tmp/video-clipper/subtitles/ dir,
so find_vtt_file could return captions from a previously-loaded video. Now
uses cache_manager::caption_cache_dir (URL-hash-keyed) with pre-download
cleanup, matching the existing per-video caching pattern.
Bump to v0.2.2.
Co-authored-by: Cursor <cursoragent@cursor.com>