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]>

README.md

@@ -336,7 +336,7 @@ Setup for Mapillary:
 
 Without the token the STREET LEVEL panel reads **KEY REQUIRED**, the Mapillary chip turns amber and the controls stay greyed out.
 
-Turn on **Street Level** under DATA LAYERS → Cameras, or open the STREET LEVEL strip in the right rail and press the **ON/OFF** pill in its header. The **PROVIDERS** row has one chip per registered provider: lighting a chip turns its imagery on (and the layer with it), and darkening the last lit chip turns the layer off. The **IMAGERY** filter (ALL / 360° / FLAT) and the **SINCE** slider (any date on the left, the last month on the right, with the cut-off date spelled out) apply to every provider at once. Mapillary's coverage draws as ground-hugging lines (recent captures bright green, older ones dim, 360° panoramas magenta); click a line for its image cones and a cone to open the photo. The photo opens at the top of the panel, so it is on screen without scrolling. The bar above it has **EXPAND** (most of the screen), **FIT** (the whole photo, with dark bars where its shape differs from the frame) / **FILL** (edge to edge, cropped) and **FOLLOW** (the globe camera moves wherever the street-level view looks; available on the Google 3D map only, since flat imagery has nothing to stand among); the caption links to the image on the provider's own site. Mapillary blurs faces and licence plates before publishing imagery, and the globe credits "© Mapillary contributors, CC BY-SA 4.0" while the provider is on.
+Turn on **Street Level** under DATA LAYERS → Cameras, or open the STREET LEVEL strip in the right rail and press the **ON/OFF** pill in its header. The **PROVIDERS** row has one chip per registered provider, in that source's colour (Mapillary green; Panoramax will be purple and Google Street View blue): lighting a chip turns its imagery on (and the layer with it), and darkening the last lit chip turns the layer off. The **IMAGERY** filter (ALL / 360° / FLAT) and the **SINCE** slider (any date on the left, the last month on the right, with the cut-off date spelled out) apply to every provider at once. Each source draws in its one colour, so Mapillary's coverage is green ground-hugging lines, overview points and image cones (360° cones are rings); click a line for its image cones and a cone to open the photo. The photo opens at the top of the panel, so it is on screen without scrolling. The bar above it has **EXPAND** (most of the screen), **FIT** (the whole photo, with dark bars where its shape differs from the frame) / **FILL** (edge to edge, cropped) and **FOLLOW** (the globe camera moves wherever the street-level view looks; available on the Google 3D map only, since flat imagery has nothing to stand among); the caption links to the image on the provider's own site. Mapillary blurs faces and licence plates before publishing imagery, and the globe credits "© Mapillary contributors, CC BY-SA 4.0" while the provider is on.
 
 The panel is also a window: drag its header to lift it out of the rail, and resize it from any edge or corner. A larger window gives the photo the room; only the settings below it scroll. Double-clicking the header, collapsing the window, or pressing SHRINK after EXPAND puts it back in the rail at its default size, so it never stays parked over the globe.