The set's concentric reps; order does not matter.
Wall-clock of the recording's first encoded frame.
The anchor, or firstFrameEpochMs when reps is empty.
Mirrors the identical calculation in the native trimmers
(VideoTrimmer.swift / VideoTrimmer.kt) so the app can commit a set the
moment the recorder finalizes its file, without waiting for the trim
re-encode to report the value back. The window starts a contextBufferMs
pad before the first rep's concentric, clamped to the first encoded frame —
the video cannot start before it began recording.
Keep in lockstep with native: a divergence here silently desynchronises the rep overlay in set review.
Wall-clock (epoch ms) of the trimmed video's first frame — the sync anchor persisted on the set as
firstFrameDate.