fix(node): measure where a seek lands instead of predicting it

The previous fix read ffprobe's key frames and took the last one at or before
the request. It was wrong twice, and a viewer felt the difference: subtitles
went from 5 s early to 2–3 s late.

Matroska's Cues index only some keyframes, so an index seek backs off to an
indexed one that the frame list does not single out. And the landing point
moves with **which streams are mapped**, because the container is positioned
where every mapped stream has data — on the reported title, a seek to 4913.7 s
landed at 4909.863 with video alone and at 4907.236 with the second audio
track mapped beside it. The frame scan gave the first number; the stream
delivered the second; the gap was 2.65 s, and the measured audio displacement
in the served stream was 2.65 s.

So the node asks ffmpeg instead: the same seek, the same mapping, one copied
frame under `-copyts`, and the answer read back off the result. 0.06–0.07 s,
cheaper than the scan it replaces. The `-ss` argument stays at the request, so
the bytes served are exactly the ones served before — only the number naming
them changes. The probe runs after the audio track is resolved, because it
cannot be right before that is known.

An answer after the request, or further before it than any real keyframe gap,
is discarded in favour of the old label: a number wrong by seconds beats a
fabricated one.

Found by decoding the served stream and locating its first frame in the
source, which put it at 4907.213 s against an announced 4909.863 s. The test
does the same thing rather than comparing the announced number against a
second reading of the same probe.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UGY17EPph5LsLzePPXhUVc

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