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/commands/require.ts

@@ -4,6 +4,7 @@
  * message the editor can show.
  */
 import { findNode, type GraphNode } from '../graph/Graph';
+import { type NodeDef, nodeDef } from '../graph/NodeRegistry';
 import type { Asset, BehaviorAsset, SceneAsset } from '../model/Asset';
 import { findObject } from '../model/hierarchy';
 import { findAsset, findScene, type Project } from '../model/Project';
@@ -41,6 +42,17 @@ export function requireNode(project: Project, assetId: string, nodeId: string):
   return node;
 }
 
+/**
+ * The definition of a node type. `NodeRegistry.requireNodeDef` throws a plain `Error`, which
+ * the editor does not catch – a command must reject an unknown type as a `CommandError` so it
+ * becomes a notice instead of a crash.
+ */
+export function requireNodeDef(type: string): NodeDef {
+  const def = nodeDef(type);
+  if (!def) throw new CommandError(`Unknown node type "${type}"`);
+  return def;
+}
+
 /** The schema entry for `key` on the asset – a property the asset's type (or the behavior) declares. */
 export function requirePropDef(asset: Asset, key: string): PropertyDef {
   const def = propDef(asset, key);