useTimelineVerticalScrollbar

Adapts generic range scrollbar state to the timeline's vertical track viewport. The hook derives a controlled `{ start, end }` value from `scrollTop`, `viewportHeight`, and track-stack bounds. Thumb changes pan the engine-owned vertical scroll position so canvas rows and external DOM chrome can stay in sync. Handle changes scale expanded track heights, giving the range handles a vertical zoom behavior that matches the horizontal viewport scrollbar's shape.