refactor: tighten terrain and sky model resource ownership

src/world/map-instancing/MapInstancingSystem.tsx

@@ -0,0 +1,42 @@
+import { Suspense } from "react";
+import { InstancedMapAsset } from "@/world/map-instancing/InstancedMapAsset";
+import {
+  MAP_INSTANCING_ASSETS,
+  type MapInstancingAssetType,
+} from "@/world/map-instancing/mapInstancingConfig";
+import { useMapInstancingData } from "@/world/map-instancing/useMapInstancingData";
+
+export function MapInstancingSystem(): React.JSX.Element | null {
+  const { data, isLoading } = useMapInstancingData();
+
+  if (isLoading || !data) {
+    return null;
+  }
+
+  const enabledAssets = Object.entries(MAP_INSTANCING_ASSETS).filter(
+    ([, config]) => config.enabled,
+  );
+
+  return (
+    <group name="map-instancing-system">
+      {enabledAssets.map(([type, config]) => {
+        const instances = data.get(type as MapInstancingAssetType);
+
+        if (!instances || instances.length === 0) {
+          return null;
+        }
+
+        return (
+          <Suspense key={type} fallback={null}>
+            <InstancedMapAsset
+              modelPath={config.modelPath}
+              instances={instances}
+              castShadow={config.castShadow}
+              receiveShadow={config.receiveShadow}
+            />
+          </Suspense>
+        );
+      })}
+    </group>
+  );
+}