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/i18n/locales/es.json

@@ -2882,7 +2882,7 @@
   "Zoom in to see street-level coverage": "Acércate para ver la cobertura a pie de calle",
   "Point the camera at the ground for street-level coverage": "Apunta la cámara al suelo para ver la cobertura a pie de calle",
   "Sequence images unavailable": "Las imágenes de la secuencia no están disponibles",
-  "Switch Street Level on to draw its coverage.": "Activa Street Level para dibujar su cobertura.",
+  "Switch Street Level on to draw its coverage.": "Activa A pie de calle para dibujar su cobertura.",
   "Every provider is switched off: light a chip to draw its coverage.": "Todos los proveedores están apagados: enciende uno para dibujar su cobertura.",
   "No sequences in view match the filter": "Ninguna secuencia a la vista coincide con el filtro",
   "No coverage in view": "No hay cobertura a la vista",

src/i18n/locales/ru.json

@@ -2949,7 +2949,7 @@
   "Zoom in to see street-level coverage": "Приблизьтесь, чтобы увидеть покрытие уличных снимков",
   "Point the camera at the ground for street-level coverage": "Направьте камеру на землю, чтобы увидеть покрытие уличных снимков",
   "Sequence images unavailable": "Снимки последовательности недоступны",
-  "Switch Street Level on to draw its coverage.": "Включите Street Level, чтобы показать покрытие.",
+  "Switch Street Level on to draw its coverage.": "Включите «Уличные снимки», чтобы показать покрытие.",
   "Every provider is switched off: light a chip to draw its coverage.": "Все источники выключены: включите один, чтобы показать его покрытие.",
   "No sequences in view match the filter": "Ни одна последовательность в кадре не подходит под фильтр",
   "No coverage in view": "В кадре нет покрытия",