Greptile note: the derive-before-apply position check could pass even if
the color call regressed to the wrong branch. Tighten the contract test
to assert exactly one label-color call, located after the marker loop's
hidden-marker continue (i.e. in the main render branch, not the hide
path), and that the once-per-frame derivation precedes it.
Marker (pin) labels previously inherited the app's default text color,
which reads white and disappears on light basemaps. Set each label's
text color in applyFrame() against the active style: black by default
(light basemaps osm-bright/positron/toner) and white only on the one
dark basemap (dark-matter). Re-evaluated every frame so a style switch
re-colors visible labels on the next applied frame.
Add a contract test pinning the dark-basemap detection and the
black-by-default / white-on-dark ternary.
Pike