fix(maps): retry the real imagery and terrain provider after a fallback

In the keyless setup a single transient failure while loading the default
map layers locked in the degraded fallback until reload.

_getImageryProvider cached the construction-fallback resolution (OSM) under
the Esri stack id, so every later "Esri Satellite" selection showed OSM.
createKeylessTerrain turns any Re:Earth failure into a flat
EllipsoidTerrainProvider; the controller cached it and set _terrainMode to
'keyless', so a boot-time network blip left the globe flat.

Fallbacks are still cached and shown at once, and no switch waits on a
retry. One controller timer now retries the real provider behind whichever
fallback is on screen (Esri's OSM stand-in, or flat terrain on a shown
globe), starting at 30 seconds and doubling to a 10-minute cap while it
keeps failing. A failed retry leaves the cached fallback and the installed
terrain untouched. A success replaces the cache entry and applies it live:
terrain is installed in place with a render request (or, behind a 3D
tileset, picked up by the next globe activation), and Esri is reselected
automatically, after any in-flight switch settles. A switch that read the
flat fallback re-reads the terrain cache before installing, and destroy()
cancels the timer and releases a provider built after it.

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+- A failed Esri Satellite or keyless terrain load still shows OSM or flat terrain
+  at once, but no longer lasts the whole session: while the fallback is on
+  screen, the real provider is retried in the background (30 seconds, doubling
+  to a 10-minute cap) and replaces it live once it loads. Map switches never
+  wait for a retry.
+
 - Add Barcelona's bike share to the GBFS registry: Bicing (543 stations, the
   city's PBSC system) and AMBici (237 e-bike stations across the metropolitan
   area, run by nextbike). Both are keyless GBFS 2.3 feeds listed in