feat(street-level): Google Street View as the second imagery provider

Google Street View registers beside Mapillary as provider `google`. It runs
on the Maps JavaScript API with the browser GOOGLE_MAPS_API_KEY, loaded on
first use (mapsLoader.js), and uses Google's own StreetViewPanorama as its
viewer adapter, emitting provider-neutral poses (address and month in the
caption, a link to Google Maps, steps along Google's arrows, FOLLOW on
Google 3D). Its chip starts off because every panorama is billed to the
key; the share option is `g`.

Google offers no coverage layer for other maps, so the provider draws
none: with it on, a click on bare ground at street zoom opens its nearest
Google-collected outdoor panorama (groundClick.js). A key Google refuses
reads KEY REJECTED and fails a pending open at once; a lookup Google never
answers fails after 10 s.

Core: providers may declare `groundClick` and `defaultOn`; poses may carry
`capturedAtPrecision` and `title`; the coverage line keeps every
provider's hint; opening one provider's image releases another's
prewarmed viewer; FIT/FILL hides for viewers without render modes. CSP
script-src admits https://maps.googleapis.com. The browser gate gains two
Street View steps (the keyed one skips as 'no Google key' in CI).

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

src/keySetupCore.mjs

@@ -36,7 +36,7 @@ export const KEY_SETUP_KEYS = Object.freeze([
   Object.freeze({
     id: 'google-maps',
     title: 'GOOGLE MAPS',
-    unlocks: 'The photorealistic 3D planet + place search',
+    unlocks: 'The photorealistic 3D planet, place search + Street View',
     getUrl: 'https://developers.google.com/maps/documentation/tile/get-api-key',
     envVars: Object.freeze(['GOOGLE_MAPS_API_KEY']),
     tier: 'metered',