refactor(maps): separate source factories and scene coordination

src/maps/availability.js

@@ -0,0 +1,15 @@
+import { keySetupRequirement } from '../keySetupCore.mjs';
+/**
+ * Why Google 3D is unavailable, phrased so the tooltip and toast recommend the
+ * RIGHT fix. With no credentials the fix is a key (or the ion route); with a
+ * key or ion token configured, the tileset failed for another reason —
+ * restrictions, quota, an EEA-billed key, or the network — and telling the
+ * user to add a key they already added is the wrong advice.
+ * @param {boolean} hasCredentials
+ * @returns {string}
+ */
+export function photorealUnavailableReason(hasCredentials) {
+  if (hasCredentials)
+    return "Google 3D tiles unavailable — check the key's API restrictions, quota, or network";
+  return `${keySetupRequirement('google-maps')} — or a Cesium ion token for the ion-hosted route`;
+}