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/registry.test.mjs

@@ -11,7 +11,6 @@ const def = (overrides = {}) => ({
   colors: { coverage: '#05cb63' },
   credit: { key: 'mapillary', html: 'Mapillary' },
   capabilities: { coverage: 'tiles', sequences: true },
-  legend: [],
   externalUrl: (id) => `https://example.test/${id}`,
   create: () => ({}),
   ...overrides,
@@ -44,7 +43,6 @@ test('every required field is checked by name', () => {
     'colors',
     'credit',
     'capabilities',
-    'legend',
     'externalUrl',
     'create',
   ])
@@ -58,7 +56,10 @@ test('every required field is checked by name', () => {
     () => validateProviders([def({ credit: { key: 'x' } })]),
     /credit/,
   );
-  assert.throws(() => validateProviders([def({ legend: 'no' })]), /legend/);
+  assert.throws(
+    () => validateProviders([def({ colors: { pano: '#f0f' } })]),
+    /coverage colour/,
+  );
   assert.throws(
     () => validateProviders([def({ pickPrefix: '' })]),
     /pick prefix/,