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]>

src/app/layers/streetLevel.js

@@ -10,12 +10,7 @@ import * as render from '../../renderGovernor.js';
 /** The browser Maps key the scene was started with (src/app/scene.js). */
 const googleApiKey = () => globalThis.window?.__GOOGLE_MAPS_API_KEY__ || null;
 
-/**
- * The default providers, once the application supplies its Street Level
- * source: Mapillary, and Google Street View, which needs only the browser
- * Maps key (off until lit). Without that source there are none, and the
- * layer reads as not configured.
- */
+/** Mapillary and Google Street View, once the Mapillary source is supplied; else none. */
 function defaultProviders(sources) {
   if (sources.mapillary == null) return [];
   return [