The header file owned value sampling, readout formatting, lane-state
resolution and the JSX at once, so a formatting change and a layout change
edited the same file. Sampling and formatting now live in
trackHeaderLaneValues, lane-state resolution in trackHeaderLaneState, and
resolveLaneHeaderState returns only the four fields its caller reads.
Also shows the track's clip count next to the track identity, which the
header promised but never rendered.
