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]>

changelog.d/974-street-level-slim.md

@@ -0,0 +1,6 @@
+- Street Level is rebuilt around one small provider registry: Mapillary,
+  KartaView, Panoramax and Google Street View share one coverage drawer and
+  one photo viewer, one source lit at a time. Floating and docked panels,
+  FOLLOW, surface modes, the legend and the Mapillary proxy's disk cache are
+  gone; `.gev-cache/mapillary/` from earlier runs is no longer read and can be
+  deleted (gekh, #974).