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/styles/foundation.css

@@ -381,7 +381,7 @@ body.cockpit-mode #cockpit-cloud-effects.active {
   position: fixed;
   right: 36px;
   bottom: 36px;
-  /* Z ladder: panels 100-139, voice pill 150, toast 200, clean-view-exit 300 */
+  /* Z ladder: panels ~100, voice pill 150, toast 200, clean-view-exit 300 */
   z-index: 150;
   display: flex;
   align-items: center;