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/selection.test.mjs

@@ -34,6 +34,7 @@ function harness({ selected = false } = {}) {
     removeListener(type, listener, options);
   };
   const sequences = { selected, cleared: 0 };
+  const groundClicks = [];
   const picks = [];
   /** What the pointer is over: a line this layer owns, or nothing. */
   const scene = { under: null };
@@ -66,6 +67,7 @@ function harness({ selected = false } = {}) {
     parts: {
       router: { ownsPick: (id) => id === 'mly:seq:1', resolve: () => null },
       hasSelectedSequence: () => sequences.selected,
+      openAtGround: (position) => groundClicks.push(position),
       clearSequences() {
         sequences.cleared++;
         sequences.selected = false;
@@ -76,6 +78,9 @@ function harness({ selected = false } = {}) {
   const onMove = state.clickHandler.getInputAction(
     Cesium.ScreenSpaceEventType.MOUSE_MOVE,
   );
+  const onClick = state.clickHandler.getInputAction(
+    Cesium.ScreenSpaceEventType.LEFT_CLICK,
+  );
   return {
     state,
     sequences,
@@ -97,6 +102,8 @@ function harness({ selected = false } = {}) {
     camera,
     picks,
     move: (x) => onMove({ endPosition: { x, y: 10 } }),
+    click: (x) => onClick({ position: { x, y: 10 } }),
+    groundClicks,
     frame() {
       for (const task of frames.splice(0)) task();
     },
@@ -256,3 +263,17 @@ test('Esc in a text field or rich-text editor keeps the selection (M61)', () =>
     h.restore();
   }
 });
+
+test('a click on bare ground goes to the ground handler; a click on anything with an id does not', () => {
+  const h = harness();
+  try {
+    h.click(5);
+    assert.deepEqual(h.groundClicks, [{ x: 5, y: 10 }]);
+    // Another layer's aircraft, or an id nobody registered: not the ground.
+    h.scene.under = 'flight:abc';
+    h.click(6);
+    assert.equal(h.groundClicks.length, 1);
+  } finally {
+    h.restore();
+  }
+});