Add multi-track DVB-TTML subtitles to HLS - #8043

Summary

• expose DVB-TTML tracks as separate WebVTT subtitle renditions in HLS
• merge subtitle renditions into FFmpeg's multitrack-audio master playlist
• synchronize WebVTT media playlists with the video variant after FFmpeg creates or rewrites its manifests
• allow generated subtitle playlists and segments through token filtering
• resolve multitrack muxer Sonar findings

Validation

• focused Community muxer/WebVTT tests
• Enterprise SRT and DVB-TTML tests
• CI unit tests, integration tests, and vulnerability checks