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/core/model/container.ts

@@ -1,7 +1,7 @@
 import { newId } from '../ids';
 import type { ContainerAsset } from './Asset';
 import { chainOf, type ObjectTree } from './hierarchy';
-import { findAsset, type Project, sceneObjectsWhere, type SceneObjectRef } from './Project';
+import { findAsset, objectsWhere, type Project, type SceneObjectRef } from './Project';
 import type { SceneObject } from './SceneObject';
 
 /** What a placed container's content object is linked to: the container and its template object. */
@@ -27,9 +27,13 @@ export function templateLinkOf(project: Project, tree: ObjectTree, object: Scene
   return undefined;
 }
 
-/** Every placed copy of a template object, across all scenes (template ids are unique per project). */
+/**
+ * Every placed copy of a template object (template ids are unique per project). Template
+ * objects themselves never carry a template id, so a template is never mistaken for a copy
+ * of itself even though the search covers container assets too.
+ */
 export function instancesOf(project: Project, templateId: string): SceneObjectRef[] {
-  return sceneObjectsWhere(project, (o) => o.templateId === templateId);
+  return objectsWhere(project, (o) => o.templateId === templateId);
 }
 
 /** Nesting depth at which a container inside a container inside … stops being expanded. */