Felix's review named "TimelineAxes" as one of four split targets.
The Y-axis and X-axis don't sit adjacent in the DOM — Y is a flex
sibling of the bars+X column — so two single-purpose components
beats a discriminator-prop component. tickIndicesFor and the
omitTickYear derivation move to TimelineXAxis where they belong.
Closes part 2 of Felix's component-split concern.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
