feat(street-level): one colour per imagery source

As more sources arrive, each one reads the same everywhere it appears: its
PROVIDERS chip, coverage lines, overview points, image cones and a single
legend swatch. The palette lives in one place, PROVIDER_COLORS in
src/layers/streetLevel/policy.js: Mapillary green (#05cb63), Panoramax
purple, Google Street View blue.

Mapillary drops its age dimming and magenta panoramas: lines and overview
points are Mapillary green, cones are green too and 360° images keep their
ring shape; the 360°/FLAT filter still separates them, and the selected
sequence stays GEV cyan.

The provider contract loses `legend`: `colors.coverage` is the source's one
colour, and the core composes the legend as one swatch per active source
(its name, its colour) followed by Selected. The chip group takes an
optional per-chip colour as a `--chip-color` CSS variable, written only
when it changes; Street Level chips show it as a swatch dot and, while on,
as the chip's border, text and a faint fill.

The feature gate checks the legend has one swatch per source plus Selected
and that the Mapillary chip carries #05cb63.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/providers/mapillary/sequences.js

@@ -103,8 +103,13 @@ export function createSequences({ state, source, parts }) {
     if (!collection) return;
     collection.removeAll();
     const heights = coneHeights(images);
-    const cone = imageConeGlyph({ size: 32, color: COLORS.image });
-    const ring = imageConeGlyph({ size: 32, color: COLORS.pano, pano: true });
+    // One colour per source; 360° images keep the ring shape.
+    const cone = imageConeGlyph({ size: 32, color: COLORS.coverage });
+    const ring = imageConeGlyph({
+      size: 32,
+      color: COLORS.coverage,
+      pano: true,
+    });
     images.forEach((image, index) => {
       if (!passesImageryFilter(image, state.filter)) return;
       const height = heights?.[index] ?? null;