Function
A position in seconds. Negative input reads as the start.
The position as MM:SS, both halves zero-padded.
Deliberately not locale-aware and deliberately not hour-aware: this is the
clock a media control shows, and MM:SS reads the same in en, es and
pt. When a duration needs words instead — "10 h 35 min" — that is
formatDuration's job, and the joining belongs in ICU messages.
Seconds are truncated rather than rounded, because a <video> reports
currentTime as a float and rounding 599.9 of a 600-second lesson up to
10:00 would claim it finished.
Format a playback position as a zero-padded
MM:SSclock.