refactor(maps): separate source factories and scene coordination

src/maps/terrain.js

@@ -0,0 +1,35 @@
+import * as Cesium from 'cesium';
+
+/** These factories are lazy: a hidden globe must not trigger terrain loading. */
+export async function createWorldTerrain(accessToken, { signal } = {}) {
+  accessToken = String(accessToken || '').trim();
+  if (!accessToken)
+    throw new Error('World terrain requires an explicit ion token');
+  signal?.throwIfAborted();
+  const resource = await Cesium.IonResource.fromAssetId(1, { accessToken });
+  signal?.throwIfAborted();
+  return {
+    provider: await Cesium.CesiumTerrainProvider.fromUrl(resource, {
+      requestVertexNormals: true,
+      requestWaterMask: false,
+      ellipsoid: Cesium.Ellipsoid.WGS84,
+    }),
+  };
+}
+
+export async function createKeylessTerrain() {
+  try {
+    // Re:Earth / Mapterhorn ellipsoidal quantized mesh, CC BY 4.0.
+    return {
+      provider: await Cesium.CesiumTerrainProvider.fromUrl(
+        'https://terrain.reearth.land/cesium-mesh/ellipsoid',
+      ),
+    };
+  } catch (error) {
+    console.warn(
+      '[MapStack] Re:Earth terrain unavailable, falling back to flat ellipsoid terrain:',
+      error,
+    );
+    return { provider: new Cesium.EllipsoidTerrainProvider() };
+  }
+}