fix(subtitle_gen): stop millisecond rounding from overflowing timestamps

_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.

tests/tools/test_subtitle_timestamps.py

@@ -0,0 +1,49 @@
+"""Regression tests: SRT/VTT timestamp formatting must not overflow milliseconds.
+
+`_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` with no carry into
+the seconds (and, at boundaries, minutes/hours) field — e.g. 0.9999s ->
+`00:00:00,1000` instead of `00:00:01,000`. Such timestamps are rejected by
+strict SRT/VTT parsers (ffmpeg subtitles filter, VLC, browser WebVTT).
+"""
+
+import re
+import sys
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from tools.subtitle.subtitle_gen import SubtitleGen  # noqa: E402
+
+_SRT_RE = re.compile(r"^\d{2}:\d{2}:\d{2},\d{3}$")
+_VTT_RE = re.compile(r"^\d{2}:\d{2}:\d{2}\.\d{3}$")
+
+
+def test_millisecond_carry_does_not_overflow():
+    # Every fractional part >= 0.9995 must carry into the next second, never
+    # emit a 4-digit millisecond field.
+    assert SubtitleGen._ts_srt(0.9999) == "00:00:01,000"
+    assert SubtitleGen._ts_vtt(0.9999) == "00:00:01.000"
+    assert SubtitleGen._ts_srt(1.9996) == "00:00:02,000"
+
+
+def test_carry_propagates_across_minute_and_hour_boundaries():
+    assert SubtitleGen._ts_srt(59.9999) == "00:01:00,000"
+    assert SubtitleGen._ts_srt(3599.9999) == "01:00:00,000"
+    assert SubtitleGen._ts_srt(7261.9999) == "02:01:02,000"
+
+
+def test_normal_values_unchanged():
+    assert SubtitleGen._ts_srt(0.0) == "00:00:00,000"
+    assert SubtitleGen._ts_srt(1.5) == "00:00:01,500"
+    assert SubtitleGen._ts_srt(0.4994) == "00:00:00,499"
+    assert SubtitleGen._ts_vtt(83.25) == "00:01:23.250"
+
+
+def test_all_outputs_are_well_formed():
+    # Sweep values that land on and around the dangerous boundary.
+    for t in (0.0, 0.4995, 0.9995, 0.9999, 1.0, 59.9995, 3599.9999, 12345.9999):
+        assert _SRT_RE.match(SubtitleGen._ts_srt(t)), SubtitleGen._ts_srt(t)
+        assert _VTT_RE.match(SubtitleGen._ts_vtt(t)), SubtitleGen._ts_vtt(t)