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/ui/chipGroup.js

@@ -28,6 +28,17 @@ export function syncChipGroup(container, chips = [], { before = null } = {}) {
       `data-toggle-chip chip-${state}${chip.active ? ' active' : ''}`,
     );
     set(button, 'textContent', chip.label);
+    // Optional per-chip colour (a Street Level source); CSS decides its use.
+    const color = chip.color || '';
+    if ((button.dataset.chipColor || '') !== color) {
+      if (color) {
+        button.style.setProperty('--chip-color', color);
+        button.dataset.chipColor = color;
+      } else {
+        button.style.removeProperty('--chip-color');
+        delete button.dataset.chipColor;
+      }
+    }
     set(button, 'title', chip.title || '');
     set(button, 'disabled', Boolean(chip.disabled));
     attribute(button, 'aria-pressed', String(Boolean(chip.active)));