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]>

docs/CODE-BOUNDARIES.md

@@ -356,8 +356,8 @@ drag or final change. Feature adapters own data mapping and service subscription
 generic modules own DOM/listeners/timers and release them in `destroy()`.
 
 Keep persistence with panel chrome: collapse uses
-`godsEyeView.v6.panelCollapsed.<panelId>`, position uses
-`godsEyeView.v8.panelPos.<panelId>`, and share state uses the registered panel id.
+`godsEyeView.v6.panelCollapsed.<panelId>`, positions are
+not stored (`godsEyeView.v8.panelPos.<panelId>` stays reserved), and share state uses the registered panel id.
 Do not rename existing
 ids or reset persistence versions when adding a body. The WEATHER adapter is
 `src/ui/weatherPanel.js`; its observed-history selection remains transient.