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/policy.js

@@ -7,6 +7,7 @@ export const POSITION_PICK_ID = 'sl:pos';
 /** One colour per imagery source: chip, coverage lines, points, cones and legend. */
 export const PROVIDER_COLORS = Object.freeze({
   mapillary: '#05cb63',
+  google: '#4285f4',
 });
 
 /** Colours every provider shares: the selection highlight and the marker. */
@@ -35,3 +36,9 @@ export const FOLLOW_MAP_STACK_ID = 'photoreal';
 
 /** Eye height above the sampled ground when the globe camera follows the viewer. */
 export const FOLLOW_EYE_HEIGHT_M = 2.4;
+
+/**
+ * A ground click opens the nearest image of a provider without coverage lines
+ * (Google Street View) only from this close: from higher up it is a misclick.
+ */
+export const GROUND_CLICK_MAX_HEIGHT_M = 3_000;