Converts WebVTT → SubRip and SubRip → WebVTT locally (direction auto-detected from the WEBVTT header). Cues are renumbered for SRT; NOTE, STYLE and REGION blocks are dropped; voice tags become "Name: " prefixes; class and timestamp tags are removed while b/i/u stay; SRT → VTT keeps identifiers, strips ASS style codes and escapes bare & and <. A millisecond offset shifts every cue (negative values clamp at zero). Malformed timing lines are reported by block number. Limit: 5,000 cues.
Positive delays subtitles; negative advances them.
keep retains <b>, <i>, <u>; strip removes all tags.