The recording clock counted setInterval ticks, which a background tab
throttles away: a recording that kept going looked frozen at 13 seconds
and was saved with that length. It now reads the wall clock instead.
MediaRecorder also writes WebM with no usable duration. Chrome omits the
element entirely, Firefox reserves a Duration of 0.0 it never fills in,
so players had no length to show and played past the end of the seek bar.
lib/webm-duration.ts stamps the recorded length into Segment > Info when
the recording stops, in place where the browser reserved room for it.
