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]>

server/providers/mapillary/tiles.js

@@ -11,7 +11,7 @@ const MAX_ZOOM = 14;
  * (positions come from the Graph API), so it is dropped in transit.
  */
 const DROP_LAYERS = ['image'];
-/** A z14 tile with its image layer is ~11 MB; anything past this is wrong. */
+/** A z14 tile with its image layer is ~12 MB; anything past this is wrong. */
 export const TILE_MAX_BYTES = 48 * 1024 * 1024;
 const FETCH_TIMEOUT_MS = 60_000;
 /** Upstream fetches at once; more wait their turn. */
@@ -260,4 +260,6 @@ export function _resetTileCacheForTest() {
   memory.clear();
   inFlight.clear();
   refusal = null;
+  upstreamActive = 0;
+  upstreamWaiting.length = 0;
 }