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]>

scripts/qa-street-level.mjs

@@ -137,11 +137,18 @@ async function main() {
           ].map((chip) => chip.dataset.chipId),
         );
         assert.deepEqual(chips, EXPECTED_PROVIDERS);
+        // One swatch per source plus "Selected"; each chip wears its colour.
         assert.equal(
           await page.evaluate(
             () => document.querySelectorAll('#sl-legend li').length,
           ),
-          4,
+          EXPECTED_PROVIDERS.length + 1,
+        );
+        assert.equal(
+          await page.$eval('[data-chip-id="mapillary"]', (chip) =>
+            chip.style.getPropertyValue('--chip-color'),
+          ),
+          '#05cb63',
         );
       },
     );