The observed-history transport is RailTimeline in src/ui/rail/, rendered
by createRailTimeline() with the same update and destroy API. Drag preview,
the coalesced 150 ms commit, release, cancel and blur behave as before; the
slider keeps the ticks it is dragged over, an update from inside a preview
cannot move the handle back, and readouts follow the handle within the same
task. utcClock moves to its own module and is still exported here.