The previous algorithm used `if (rem >= DROP)` (i.e. rem >= 4) to decide
whether to advance to the next minute group. This fired immediately at
frame 4, still inside minute 0 of the 10-minute non-drop group, producing
00:01:00;00 for what should be 00:00:00;04. Every timecode display in
the editor was wrong for any position past the first four frames.
Each 10-minute block has one 3600-frame non-drop minute followed by nine
3596-frame drop minutes. The fix checks `rem < FRAMES_FIRST_MIN` (3600)
to identify the non-drop minute, then subtracts it before dividing into
drop-minute slots. Frame labels within drop minutes are shifted by DROP
(+4) so the first usable label is :00;04 as per SMPTE 12M.
