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]>

docs/CURRENT-STATE.md

@@ -3033,8 +3033,9 @@ shared 360°/flat and captured-since filters (relative days, so a link keeps its
 meaning), one viewer host and one on-globe credit per active provider. Each
 provider draws in one colour (`PROVIDER_COLORS` in
 `src/layers/streetLevel/policy.js`); 360° cones are rings and the selected
-sequence is GEV cyan. Share options: `m` (Mapillary), `p` (`a`/`p`/`f`
-panoramas), `s` (since, days). Only Mapillary is registered. Without
+sequence is GEV cyan. Share options: `m` (Mapillary), `g` (Google Street
+View), `p` (`a`/`p`/`f` panoramas), `s` (since, days). Mapillary and Google
+Street View are registered. Without
 `MAPILLARY_CLIENT_TOKEN` the panel reads KEY REQUIRED and the controls are
 disabled. With it, coverage draws as z0–5 overview points from orbit and z11–14
 sequence lines below 60 km; the proxy strips the unused `image` layer from z14
@@ -3080,8 +3081,23 @@ manages each provider's credit and fans the filter out to every provider. To
 add a provider: implement the definition, register it in
 `src/app/layers/streetLevel.js`, add its boolean option to the `street-level`
 group in `src/data/layerState.js` and its modules to
-`scripts/package-boundaries.json`. Once a keyless provider registers, the
-layer's `requiresKeyId` becomes null and the key gate moves to the chips.
+`scripts/package-boundaries.json`. With providers on different keys the
+layer's `requiresKeyId` is null and each chip reports its own key.
+
+Google Street View (`providers/google/`) runs on the Maps JavaScript API with
+the browser `GOOGLE_MAPS_API_KEY`, which needs that API enabled; it is loaded
+on first use (`mapsLoader.js`, CSP `script-src https://maps.googleapis.com`).
+Its chip starts off (`defaultOn: false`, option default `g` off) because every
+panorama load is billed to the key. Google offers no coverage layer for
+other maps, so it draws none: with it on, a click on bare ground (nothing
+picked, so a Mapillary line under the pointer wins; no tool holding the
+pointer; camera below 3 km) opens its nearest
+Google-collected outdoor panorama within 50 m (`groundClick.js`). The viewer
+is Google's `StreetViewPanorama` in its own element in the viewer host,
+without Google's fullscreen, close and address controls (the address goes in
+the caption); its poses carry the month the panorama was taken. A key Google
+refuses (`gm_authFailure`) reads KEY REJECTED and fails a pending open at
+once. The gate's Street View steps skip with `no Google key` in CI.
 
 Street Level has two browser gates. `npm run qa:street-level -- --url <server>`
 runs against real Mapillary and needs `MAPILLARY_CLIENT_TOKEN`. `npm run