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]>

scripts/qa-ui-disposal.mjs

@@ -43,11 +43,7 @@ try {
     const fpsReadout = document.querySelector('.frame-rate-readout');
     const fpsWasVisible = fpsReadout && !fpsReadout.hidden;
     const watchObserver = (name) => {
-      const owner =
-        name === '_draggableResizeObserver'
-          ? ui._panelPosition
-          : ui._panelLayout;
-      const observer = owner[name];
+      const observer = ui._panelLayout[name];
       if (!observer) return false;
       const disconnect = observer.disconnect.bind(observer);
       counts[name] = 0;
@@ -59,7 +55,6 @@ try {
     };
     const observed = [
       '_commandDockTrayObserver',
-      '_draggableResizeObserver',
       '_leftStackResizeObserver',
       '_rightStackResizeObserver',
       '_leftStackMutationObserver',
@@ -193,11 +188,7 @@ try {
               counts[field] === 1 && ui._contextControls[field] === null,
           ),
         observersReleased: observed.every(
-          (name) =>
-            counts[name] === 1 &&
-            (name === '_draggableResizeObserver'
-              ? ui._panelPosition
-              : ui._panelLayout)[name] === null,
+          (name) => counts[name] === 1 && ui._panelLayout[name] === null,
         ),
         subscriptionReleased:
           counts.cctv === 1 && ui._cctvControls._cctvUnsubscribe === null,