`dvd_time_t` stores H:M:S:F as non-drop-frame timecode with a rate flag,
not elapsed wall-clock time. On NTSC the seconds field advances every 30
frames, but a frame lasts 1001/30000 s, so 30 frames occupy 1.001 s of
real time. Reading H:M:S as literal seconds under-reports real time by
exactly 0.1% — 3.6 s per hour, growing with elapsed time, which is what
made chapter marks drift ~4 s by the 67-minute mark.
Convert the whole timecode through an integer frame count and apply the
exact 1001/30000 fraction once, instead of reading H:M:S literally and
dividing only the frame remainder by a decimal 29.97. Chapter marks sum
per-cell frame counts as integers and convert once per chapter, so every
mark lands on an exact frame boundary instead of accumulating f64
rounding across a long title.
PAL is arithmetically unaffected: 25 frames = 1.000 s exactly, so the
old and new paths agree to within one ULP. Sweeping all 900k H:M:S:F
combinations under 10 hours, 12 differ, by at most 8.9e-16 s — the new
path divides one exact integer instead of adding a rounded fraction to a
large one, so where they differ it is the more accurate of the two.
Five existing NTSC fixtures asserted the old literal-seconds values and
were updated to real seconds (each is exactly 1.001x its old figure);
`bcd_secs` now documents that its argument is timecode, not real time.
Fixes#1
Reported-by: AnimeFN <admin@animefn.com>
