feat: high-contrast cluster markers over choropleths

[joaquimds]

Closes COOP-3729 — https://linear.app/commonknowledge/issue/COOP-3729/white-markers-when-data-viz-is-active-for-better-visibility

What

When a choropleth fill is painted, cluster markers switch to a high-contrast style: a white disc with a 1px edge in the marker colour, and the count drawn in the marker colour. Pin strokes and icon halos widen from 1px to 2px. When no choropleth is painted, markers are unchanged.

Pale marker colours are darkened (by luminance, preserving hue) for the edge and count so they still read against white.

Why not the per-scheme rules in the issue

• The clash depends on both the choropleth colour and the marker colour, and marker colour is configurable per data source and view. A list keyed on scheme alone can't cover it (and misses Sequential blues vs the default blue member colour).
• White-middle schemes fail white markers exactly where data is mid-range; blues fail them at the pale end. A data-density heuristic would flip styles as filters change and still only approximates what's on screen after opacity and basemap blending.
• A white disc with a coloured edge is two-tone, so at least one tone contrasts with any background.

Changes

• ClustersLayer.tsx, PinsLayer.tsx: new onChoropleth flag branches the paint properties.
• Markers.tsx: derives the flag from the choropleth toggle, a selected boundary set and loaded area stats; passed through DataSourceMarkers. Applies to public maps too.
• src/utils/colors.ts: getContrastingRingColor and a shared parseRgb; colorWithAlpha now uses it.
• tests/unit/utils/colors.test.ts: covers the parser, alpha helper and darkening.

Testing

• npm run lint clean; npm test -- run tests/unit/utils/colors.test.ts passes.
• Manually check red-blue, blues and a categorical choropleth with the toggle on and off, zoomed past the cluster zoom, and a public map with clustering.

🤖 Generated with Claude Code

[linear-code]

COOP-3729