_ts_srt/_ts_vtt computed the seconds and millisecond fields independently:
`ms = int(round((seconds % 1) * 1000))`. When the fractional part is >= 0.9995
that rounds to 1000, emitting a malformed 4-digit `…,1000` value with no carry
into the seconds field (and, at 59.9999/3599.9999, no carry into minutes/hours).
For example 0.9999s became `00:00:00,1000` instead of `00:00:01,000`. ASR word
and segment end-times routinely land on such fractional boundaries, and the
resulting cue is rejected or mistimed by strict SRT/VTT parsers (ffmpeg
subtitles filter, VLC, browser WebVTT).
Decompose from a single rounded total-milliseconds value so the carry
propagates across all fields. Both formatters now share one `_hmsms` helper.
bcd8eb6e53