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]>

CHANGELOG.md

@@ -8,6 +8,8 @@
 
 - Street Level: a provider-neutral street-level imagery layer modelled on the iD editor's photo overlay. One panel in the right rail carries a chip per imagery provider, shared 360°/flat and captured-since filters, coverage drawn on the globe from orbit down to a single street, image cones per sequence, and one embedded viewer whose adapter follows the provider of the open image; each active provider credits its imagery on the globe, and share links carry the provider switches and the filter. Mapillary is the first provider (free client token; CC BY-SA 4.0 imagery); Google Street View, KartaView and Panoramax are next. The panel is kept simple: the viewer opens at the top so the photo needs no scrolling, the header pill and the provider chips switch the layer, and SINCE is a slider. FOLLOW (the globe camera following the street-level view) is available on the Google 3D map only.
 
+- Street Level draws each imagery source in one colour, everywhere it appears: its PROVIDERS chip, coverage lines, overview points, image cones and a single legend swatch. Mapillary is green (no more age dimming or magenta panoramas; 360° cones are rings, and the 360°/FLAT filter still separates them); Panoramax will be purple and Google Street View blue.
+
 - Street Level coverage no longer floats over trees and bridges on the Google 3D map. At street zoom, sequence lines, image cones and the position marker are placed on the bare earth from the terrain service (with a 2 m lift) instead of being draped on the top of the photoreal mesh, so buildings and tree canopies hide what is behind them; other maps and higher views keep draped lines. Opening a photo no longer flies the camera underground when the Google tiles under it have not loaded yet.
 
 - CCTV cameras whose bearing is a guess now say so. Packs mark bearings derived