refactor: share timeline geometry and presentation formatting

Gantt and static Timeline now use one prepared lane plot. React retains native keyed lane/marker controls across clock updates, keeps at most a viewport of rows mounted, and positions tooltips from the current viewport. Dense marker hit regions exclude transparent strokes; a queued scroll preserves a tooltip already placed at the new position and clears stale geometry on a real scroll.

Run ID: run_d6a451eac952679e802d772b0104ffcb