feat(street-level): one imagery source at a time; IC8 review fixes

- One provider is lit at a time: lighting a chip darkens the others, and a
  chip click records every registered provider's switch in one update so
  the share link keeps the swap (also off Google 3D, where Street View is
  not listed). The layer starts with the first default-on provider
  (Mapillary, free); params that light several keep the one already on,
  so reapplying them never flips (litByParams). The now-unreachable
  "several lit" chip paths are gone.
- Leaving Google 3D during a Street View lookup ends it without an error
  or a stuck loading state (openImage's early return resets it; the
  lookup loop rechecks availability).
- A source swap during a lookup no longer reports "no imagery".
- Street View lit off Google 3D says the panel needs the Google 3D map.
- Comments trimmed to the why; keySetupCore.test.mjs back to upstream's
  formatting plus the one new test (was a 551-line reformat).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/CURRENT-STATE.md

@@ -3108,11 +3108,11 @@ miss cooldown. Roads on elevated decks are still drawn at
 ground level. Framing a photo ignores mesh samples far below the bare earth
 (unloaded tiles).
 
-The header pill is the layer switch. With one provider its chip is a layer
-switch too; with several, darkening the last lit chip turns the layer off. A
-chip without a usable key explains the key instead of lighting (or of
-switching the layer off as the last lit chip); it still darkens beside a lit
-one, and the other chips keep switching. When the switched-on providers lack
+The header pill is the layer switch. Lighting a chip swaps the source;
+clicking the lit chip turns the layer off. A chip without a usable key
+explains the key instead of switching, and the other chips keep switching.
+Lit off its map stack, a provider leaves the panel saying which map it
+needs. When the switched-on providers lack
 their keys but a switched-off one would draw with a key already set (no
 Mapillary token, Street View on Google 3D with a Google key), the layer
 reports it (`keyAlternatives`), and the panel's KEY REQUIRED, its coverage
@@ -3158,7 +3158,14 @@ and leaves the camera where it is (`groundClick.js`). Higher up the hint
 says to zoom in. **OPEN STREET VIEW** beside that hint (`#sl-nearest-btn`)
 does the same at the view centre, so Street View needs no pointer; it is
 disabled until the camera is at street zoom (and aimed at the sky, it opens
-nothing and says to aim at a street). The chip reads GOOGLE STREET VIEW. Its
+nothing and says to aim at a street). One provider is lit at a time:
+`setProviderEnabled(id, true)` darkens the others, a chip click sends every
+switch in one update so the share link records the swap, and params that
+light several keep the one already on, else the first in chip order
+(`litByParams`; reapplying them never flips). Mapillary, free, is lit by
+default. The chip reads GOOGLE STREET VIEW, and an open panorama's globe
+marker is a Google-blue view cone (`marker: {kind: 'view-cone'}`) in place
+of GEV's amber pin. Its
 panoramas are all 360° and it draws no coverage, so it declares `filters: []`
 (a provider's `filters` default to both `pano` and `since`): lookups ignore
 the IMAGERY and SINCE filters, the panel hides each filter row no lit provider