Enable dual subtitles

This closes !1654.

This PR depends on and should be merged together with VLCKit!371 (merged) and VLCMediaLibraryKit!62 (merged).

Add dual subtitle support for primary and secondary tracks

• Add separate tracking for primary/secondary subtitle indices in VLCPlaybackService
• Implement syncVideoSubtitleState for coordinated subtitle management
• Store both primary and secondary subtitle track indices in VLCMLMedia for per-media persistence
• Split TitleSelectionView into separate Primary/Secondary subtitle sections
• Add conflict resolution when same track selected for both streams

Enables simultaneous display of two subtitle tracks for language learning scenarios.