Support foliage layers in project scene creation and validate layer enabled status

src/document/scene-document.ts

@@ -270,7 +270,12 @@ export function createEmptyProjectScene(
   overrides: Partial<
     Pick<
       ProjectScene,
-      "id" | "name" | "loadingScreen" | "editorPreferences" | "world"
+      | "id"
+      | "name"
+      | "loadingScreen"
+      | "editorPreferences"
+      | "world"
+      | "foliageLayers"
     >
   > = {}
 ): ProjectScene {
@@ -286,7 +291,7 @@ export function createEmptyProjectScene(
     world: overrides.world ?? createDefaultWorldSettings(),
     brushes: {},
     terrains: {},
-    foliageLayers: createEmptyFoliageLayerRegistry(),
+    foliageLayers: overrides.foliageLayers ?? createEmptyFoliageLayerRegistry(),
     paths: {},
     modelInstances: {},
     entities: {},

src/foliage/foliage.ts

@@ -318,6 +318,10 @@ export function cloneFoliageLayer(layer: FoliageLayer): FoliageLayer {
     );
   }
 
+  if (typeof layer.enabled !== "boolean") {
+    throw new Error("Foliage layer enabled must be a boolean.");
+  }
+
   return {
     id: normalizeNonEmptyString(layer.id, "Foliage layer id"),
     name: normalizeNonEmptyString(layer.name, "Foliage layer name"),