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/layers/streetLevel/selection.js

@@ -21,7 +21,11 @@ export function createSelection({ state, parts }) {
       ? picking.resolvePickId(picked)
       : picked?.id;
     const route = parts.router.resolve(id);
-    if (!route?.instance) return;
+    // Nothing of ours, nor anyone else's: the ground itself.
+    if (!route?.instance) {
+      if (!id) parts.openAtGround?.(click.position);
+      return;
+    }
     const entry = state.providers.get(route.providerId);
     if (!entry?.on) return;
     route.instance.handlePick(route.id);