Fix findUsages/undo/node-lookup bugs, split Viewport and Editor, share command and inspector helpers

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/runtime/World.ts

@@ -4,6 +4,7 @@ import type { Asset } from '../core/model/Asset';
 import { isPlaceableType } from '../core/model/AssetTypes';
 import { instantiateContainer } from '../core/model/container';
 import { findObject } from '../core/model/hierarchy';
+import { descendantsBelow } from '../core/model/walk';
 import { findAsset, findScene, type Project } from '../core/model/Project';
 import { createSceneObject, type SceneObject } from '../core/model/SceneObject';
 import { footprintContains } from '../core/properties/bounds';
@@ -71,7 +72,7 @@ export class World {
     const keep = new Set<Entity>();
     for (const object of scene.objects) {
       const asset = findAsset(this.project, object.assetId);
-      if (!asset || asset.type === 'scene') continue;
+      if (!asset || !isPlaceableType(asset.type)) continue;
       keep.add(this.materialize(object, asset));
     }
     for (let i = this.entities.length - 1; i >= 0; i--) if (!keep.has(this.entities[i]!)) this.entities.splice(i, 1);
@@ -121,7 +122,7 @@ export class World {
     entity.parent = parentEntity;
     for (const child of created.slice(1)) {
       const childAsset = findAsset(this.project, child.assetId);
-      if (!childAsset || childAsset.type === 'scene') continue;
+      if (!childAsset || !isPlaceableType(childAsset.type)) continue;
       this.materialize(child, childAsset).parent = this.findByObjectId(child.parentId) ?? null;
     }
     this.structureChanged = true;
@@ -144,7 +145,7 @@ export class World {
     for (let i = scene.objects.length - 1; i >= 0; i--) {
       const object = scene.objects[i]!;
       const asset = findAsset(this.project, object.assetId);
-      if (asset && asset.type !== 'scene' && !alive.has(object.id)) scene.objects.splice(i, 1);
+      if (asset && isPlaceableType(asset.type) && !alive.has(object.id)) scene.objects.splice(i, 1);
     }
     for (const entity of this.entities) {
       const object = findObject(scene, entity.objectId);
@@ -219,15 +220,7 @@ export class World {
 
   /** Everything parented below an entity, at any depth. */
   descendantsOf(entity: Entity): Entity[] {
-    const result: Entity[] = [];
-    const visit = (parent: Entity) => {
-      for (const child of this.childrenOf(parent)) {
-        result.push(child);
-        visit(child);
-      }
-    };
-    visit(entity);
-    return result;
+    return descendantsBelow(entity, (parent) => this.childrenOf(parent), (child) => child);
   }
 
   findByObjectId(objectId: string): Entity | undefined {