Shift subtitle timing and convert basic SRT / WebVTT

Deliver a UTF-8 text file containing subtitle_text converted to target_format (srt or vtt), with every start/end timestamp shifted by integer shift_ms. Preserve cue text and cue order, normalize line endings to LF, regenerate SRT sequence numbers and omit original cue identifiers. Output is a .txt file that can be renamed to .srt or .vtt. Input must be basic SRT or WebVTT, at most 1 MB UTF-8 and 5,000 cues, without styles, notes, metadata or positioning settings. Offset is within plus/minus 86,400,000 ms; resulting start times must be nonnegative and ends after starts. No transcription or translation.