fix(street-level): /revmain cleanup after the slim rebuild

- Google 3D: the OPEN button re-measures a ground whose tiles are still
  loading (a few times, a second apart) and on a map change, not only
  after the next camera move.
- Mapillary: two mounts on one host during the first download share one
  build instead of leaking a second WebGL viewer.
- Retired floating-panel keys (v8 panelPos, panelFloatHintShown) are
  dropped from localStorage; dead inline-position resets, the drag JSDoc
  and the dead qa-ui-disposal observer branch are gone.
- Docs: z ladder, panel chrome, position storage and the fixture gate's
  actual coverage; changelog fragment for #974 (old .gev-cache/mapillary
  can be deleted).
- ru/es: the layer is named as everywhere else in the UI.
- Fixtures and test fakes: unused exports removed; a fake credit removal
  of an absent credit no longer drops another one; the tile test reset
  frees upstream slots.

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

src/layers/streetLevel/viewerHost.test.mjs

@@ -385,6 +385,19 @@ test('a Mapillary viewer unmounted mid-download is never built; the next mount b
   assert.equal(viewers.live, 0, 'nothing left holding a WebGL context');
 });
 
+test('two mounts on one host during the first download build one Mapillary viewer', async () => {
+  const { gate, viewers } = fakeLibrary();
+  const adapter = createMapillaryViewer({ source: { token: 't' } });
+  const element = {};
+  const first = adapter.mount(element);
+  const second = adapter.mount(element);
+  gate.resolve();
+  await Promise.all([first, second]);
+  assert.equal(viewers.created, 1);
+  adapter.unmount();
+  assert.equal(viewers.live, 0, 'nothing left holding a WebGL context');
+});
+
 test('closing the photo stops sequence playback in the hidden viewer', async () => {
   const { gate, viewers } = fakeLibrary();
   gate.resolve();