Video track + clip selection — ED.9

The video lane shows the recording as its segments — proportional clip blocks along the timeline — and lets you select one to edit.

segment_spans is the pure layout: each TimelineSegment becomes a start_fraction / width_fraction of the project, so the lane tiles responsively at any width. A 2× segment is half as wide as its source span (it occupies half the project time) — width tracks project length, not source length, which is what keeps the lane in sync with the ruler after a speed change.

The VideoFilmstrip component renders those spans as clip blocks with duration labels; clicking one sets the selected-clip signal (a RwSignal<Option<usize>> in context) that the inspector (ED.18) and edit operations (ED.11) read. Because the spans are derived from the segment list every render, the lane re-flows automatically when a split or trim changes the segments.