Function BuildWebVtt

Builds a WebVTT document (WEBVTT\n...) from master-timeline narration lines.

Grouping rules for lines with WordTimings:

• Start a new cue after sentence-ending punctuation (., ?, !), at 7 words, or at 42 characters.
• Each cue spans from its first word's StartSeconds to its last word's EndSeconds.
• Lines without WordTimings emit a single fallback cue spanning StartSeconds to StartSeconds + DurationSeconds.
• Empty input returns "WEBVTT\n".