Some services use WebVTT files with:
- Cue identifiers (Q0, Q1, etc.) before timing lines that pysubs2/pycaption incorrectly parses as subtitle text
- Multi-line subtitles split into separate cues with 1ms offset times and different line: positions (e.g., line:77% for top, line:84% for bottom)
Added detection and sanitization functions:
- has_webvtt_cue_identifiers(): detects cue identifiers before timing
- sanitize_webvtt_cue_identifiers(): removes problematic cue identifiers
- has_overlapping_webvtt_cues(): detects overlapping cues needing merge
- merge_overlapping_webvtt_cues(): merges cues sorted by line position
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