Merge upstream/main into pr/google-street-view

Conflicts with upstream's Street Level provider composition and share-state
codec, resolved:

- Street View's switch moves from its own share option (`g`) to upstream's
  provider-switch field (`r`, e.g. `0.r.google-1`); a link without it keeps
  Street View off. Docs and the browser gate follow.
- createApplicationStreetLevel keeps upstream's injectable provider list;
  its default, once the application supplies its Street Level source, is
  Mapillary plus Google Street View. Without that source there are none, as
  upstream intends.
- The coverage hint keeps upstream's empty-composition message and joins
  every switched-on provider's hint otherwise.
- CHANGELOG, README and SECURITY keep both sides' entries.

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

src/layers/streetLevel/index.test.mjs

@@ -665,3 +665,17 @@ test('with no Mapillary token, the layer points at Street View, on Google 3D onl
   layer.setProviderEnabled('google', true);
   assert.equal(layer.getStats().keyRequired, false);
 });
+
+test('an empty provider composition enables and tears down without acquiring imagery', async (t) => {
+  const { layer, viewer } = await enabledLayer(t, []);
+  assert.deepEqual(layer.providerIds, []);
+  assert.deepEqual(layer.getUIState().providers, []);
+  assert.equal(layer.getUIState().keyRequired, false);
+  assert.match(
+    layer.getUIState().coverage.hint,
+    /No street-level imagery providers/,
+  );
+  layer.disable();
+  layer.enable(viewer);
+  assert.deepEqual(layer.getUIState().providers, []);
+});