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/groundClick.js

@@ -0,0 +1,31 @@
+import { GROUND_CLICK_MAX_HEIGHT_M } from './policy.js';
+import { cameraHeightAboveGround, groundPointAt } from './view.js';
+
+/**
+ * A click on bare ground at street zoom opens the nearest image of the active
+ * providers that have no coverage to click (`groundClick`, Google Street
+ * View). `openNearest(point, {providerIds})` is the layer's lookup.
+ * @returns {(screenPosition: {x: number, y: number}) => Promise<boolean>|false}
+ */
+export function createGroundClick({ state, parts, openNearest }) {
+  return function openAtGround(screenPosition) {
+    const isWorldPosition = state.services.scenePick?.isPickedWorldPosition;
+    if (!state.enabled || !state.viewer || !isWorldPosition) return false;
+    const providerIds = [...state.providers.values()]
+      .filter(
+        (entry) =>
+          entry.on && entry.def.groundClick && entry.status?.configured,
+      )
+      .map((entry) => entry.def.id);
+    if (!providerIds.length) return false;
+    // From higher up a click is a misclick, not a street.
+    const height = cameraHeightAboveGround(state.viewer, {
+      groundAt: parts.groundCaster?.groundAt,
+    });
+    if (!(height <= GROUND_CLICK_MAX_HEIGHT_M)) return false;
+    const point = groundPointAt(state.viewer, screenPosition, {
+      isWorldPosition,
+    });
+    return point ? openNearest(point, { providerIds }) : false;
+  };
+}