feat(street-level): OPEN STREET VIEW, and Street View only on Google 3D

- OPEN STREET VIEW in the PROVIDERS row opens Street View's nearest
  panorama at the ground under the middle of the screen, without moving
  the camera: Street View no longer needs a pointer. It is disabled (and
  says so) until the camera is at street zoom.
- Off the Google 3D map Street View does not appear at all: no chip,
  legend swatch, globe credit, layer source or button, and Google's script
  is not loaded (the viewer has no prewarm; the script loads on first use).
  The share option keeps the user's choice for when Google 3D returns.
- A ground click measures the camera height at the click rather than
  reading the cached flag.
- A chip without a usable key explains it instead of lighting; it darkens
  beside a lit chip, and the other chips keep switching, so a refused
  Street View key cannot lock Mapillary out.
- The gate checks that nothing of Street View shows off Google 3D even when
  a share link switches it on, presses OPEN STREET VIEW on Google 3D, and
  asserts that each chip it switches really switched.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/CURRENT-STATE.md

@@ -3060,7 +3060,10 @@ ground level. Framing a photo ignores mesh samples far below the bare earth
 (unloaded tiles).
 
 The header pill is the layer switch. With one provider its chip is a layer
-switch too; with several, darkening the last lit chip turns the layer off. The
+switch too; with several, darkening the last lit chip turns the layer off. A
+chip without a usable key explains the key instead of lighting (or of
+switching the layer off as the last lit chip); it still darkens beside a lit
+one, and the other chips keep switching. The
 viewer sits under the header with EXPAND, FIT/FILL, FOLLOW and close above the
 image. FOLLOW needs the Google 3D map stack (`attachMapStackController`) and
 stops when the stack changes. Camera moves go through the application's
@@ -3090,14 +3093,18 @@ 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's terms forbid Street View beside
 non-Google maps, so the provider declares `requiresMapStack: 'photoreal'`:
-off Google 3D its chip is dashed with the reason, lookups and opens skip it,
-and leaving Google 3D closes its panorama. Google offers no coverage layer
+off Google 3D it does not appear at all (no chip, legend swatch, globe
+credit, layer source or button, and Google's script is not loaded), lookups
+and opens skip it, and leaving Google 3D closes its panorama; the share
+option keeps the user's choice for when Google 3D returns. Google offers no coverage layer
 for other maps, so it draws none: with it on, a click on the map itself
 (`scenePick.isSurfacePick`: nothing picked or Google 3D tile content, 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
 and leaves the camera where it is (`groundClick.js`). Higher up the hint
-says to zoom in. The viewer is Google's `StreetViewPanorama` in its own
+says to zoom in. **OPEN STREET VIEW** in the PROVIDERS row (`#sl-nearest-btn`)
+does the same at the view centre, so Street View needs no pointer; it is
+disabled until the camera is at street zoom. 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); a pose goes out at once and
 again when the panorama's month, photographer and address arrive (one
@@ -3105,8 +3112,9 @@ lookup per panorama per session, seeded by the nearest lookup). A key
 Google refuses (`gm_authFailure`: the API not enabled, a referrer it does
 not allow, billing off) reads KEY REJECTED and fails a pending open at
 once; a lookup or open Google never answers fails after 10 or 20 s. In CI
-(no Google key, no Google 3D) only the chip and share-link step runs; the
-click step skips as `no Google key`.
+(no Google key, no Google 3D) the gate checks that nothing of Street View
+shows off Google 3D, even switched on by a share link; the click and button
+step skips as `no Google key`.
 
 Street Level has two browser gates. `npm run qa:street-level -- --url <server>`
 runs against real Mapillary and needs `MAPILLARY_CLIENT_TOKEN`. `npm run