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/testSupport/streetLevelFakes.mjs

@@ -205,8 +205,11 @@ export function fakeCesiumViewer({ view = false } = {}) {
     camera,
     creditDisplay: {
       addStaticCredit: (credit) => credits.push(credit),
-      removeStaticCredit: (credit) =>
-        credits.splice(credits.indexOf(credit), 1),
+      removeStaticCredit: (credit) => {
+        // Cesium ignores a credit it does not hold.
+        const at = credits.indexOf(credit);
+        if (at !== -1) credits.splice(at, 1);
+      },
     },
   };
 }