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/ui/panelPositionControls.js

@@ -8,6 +8,11 @@ const PANEL_LAYOUT_STORAGE_VERSION = 'v6';
  * the layout-reset notice and the documented key.
  */
 const PANEL_POSITION_STORAGE_VERSION = 'v8';
+/** Floating panels' stored positions and hint flag: nothing reads them now. */
+const RETIRED_PANEL_KEY_PREFIXES = [
+  `godsEyeView.${PANEL_POSITION_STORAGE_VERSION}.panelPos.`,
+  `godsEyeView.${PANEL_POSITION_STORAGE_VERSION}.panelFloatHintShown`,
+];
 export class PanelPositionControls {
   constructor({ syncPanelCollapseButton, showToast }) {
     this._syncPanelCollapseButton = syncPanelCollapseButton;
@@ -15,6 +20,7 @@ export class PanelPositionControls {
   }
 
   _maybeNotifyLayoutReset() {
+    this._dropRetiredPanelKeys();
     try {
       const marker = `godsEyeView.${PANEL_POSITION_STORAGE_VERSION}.layoutResetNotified`;
       if (localStorage.getItem(marker)) return;
@@ -32,6 +38,16 @@ export class PanelPositionControls {
     }
   }
 
+  _dropRetiredPanelKeys() {
+    try {
+      for (const key of Object.keys(localStorage))
+        if (RETIRED_PANEL_KEY_PREFIXES.some((prefix) => key.startsWith(prefix)))
+          localStorage.removeItem(key);
+    } catch {
+      // storage unavailable
+    }
+  }
+
   _panelCollapseStorageKey(panelId) {
     return `godsEyeView.${PANEL_LAYOUT_STORAGE_VERSION}.panelCollapsed.${panelId}`;
   }