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/viewerHost.js

@@ -33,6 +33,9 @@ export function createViewerHost({ state, parts }) {
       altitude: Number.isFinite(pose.altitude) ? pose.altitude : null,
       isPano: pose.isPano === true,
       capturedAt: pose.capturedAt ?? null,
+      capturedAtPrecision:
+        pose.capturedAtPrecision === 'month' ? 'month' : 'day',
+      title: pose.title || null,
       sequenceId: pose.sequenceId ?? null,
       creator: pose.creator || null,
       externalUrl: pose.externalUrl || null,
@@ -68,7 +71,15 @@ export function createViewerHost({ state, parts }) {
       active.adapter.unmount();
       active = null;
     }
+    // Another provider's prewarmed viewer shares the host: it would sit
+    // under this one, holding its WebGL context.
+    for (const [id, warm] of warmed) {
+      if (id === entry.def.id) continue;
+      warmed.delete(id);
+      warm.unmount();
+    }
     const adapter = entry.instance.viewer;
+    state.street.renderModes = typeof adapter.setRenderMode === 'function';
     const promise = (async () => {
       await adapter.mount(state.street.host);
       if (mounting?.promise !== promise) {
@@ -183,6 +194,8 @@ export function createViewerHost({ state, parts }) {
     for (const entry of entries) {
       const adapter = entry.instance.viewer;
       if (!adapter.prewarm || !entry.on) continue;
+      // Another provider's viewer is showing in the host: do not build under it.
+      if (active && active.id !== entry.def.id) continue;
       try {
         await adapter.prewarm(host);
       } catch {