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/fixtures/street-level/kartaView.mjs

@@ -49,11 +49,6 @@ export function resetKartaViewFixture() {
   nearestLookups = 0;
 }
 
-/** Nearest lookups answered since the last reset (the first got a 429). */
-export function kartaViewLookups() {
-  return nearestLookups;
-}
-
 /** The drive's photos, south to north; every tenth is 360°, every odd one archived. */
 export function kartaViewPhotos() {
   const step = KV_SPACING_M / METRES_PER_DEG_LAT;