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]>

docs/CURRENT-STATE.md

@@ -2778,7 +2778,11 @@ collapsible panel (`#street-level-panel`, share token `0`, option owner
 overlay: one panel, a PROVIDERS chip per registered imagery provider, shared
 360°/flat and captured-since filters (stored as relative days so a link keeps
 its meaning), one viewer host, and one on-globe credit per active provider.
-Share options: `m` (Mapillary on/off), `p` (a/p/f panorama mode), `s`
+Each source has one colour (`PROVIDER_COLORS` in
+`src/layers/streetLevel/policy.js`: Mapillary green `#05cb63`, Panoramax
+purple, Google Street View blue), used for its chip, coverage lines, overview
+points, image cones and its single legend swatch; 360° cones are rings, and
+the selected sequence is GEV cyan. Share options: `m` (Mapillary on/off), `p` (a/p/f panorama mode), `s`
 (since, days). Only Mapillary is registered in this build. Without
 `MAPILLARY_CLIENT_TOKEN` the header reads KEY REQUIRED, the Mapillary chip
 turns amber and the controls stay disabled; with it, coverage draws as
@@ -2831,8 +2835,8 @@ the whole photo and caption fit without scrolling.
 
 Street-level providers implement the contract documented in
 `src/layers/streetLevel/registry.js`: a definition (`id`, `name`, `label`,
-`requiresKeyId`, `pickPrefix`, `colors`, `credit`, `capabilities`, `legend`,
-`externalUrl`, `create`) whose `create(context)` returns an instance with
+`requiresKeyId`, `pickPrefix`, `colors` (`{coverage}`, the source's one
+colour), `credit`, `capabilities`, `externalUrl`, `create`) whose `create(context)` returns an instance with
 `status`, `init`/`activate`/`deactivate`/`destroy`, `refreshCoverage`,
 `setFilter`, optional `setSurface`, `coverageStats`, `handlePick`, optional
 sequence selection,