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/graph/NodeContext.ts

@@ -1,4 +1,5 @@
 import type { PropertyBag } from '../model/Properties';
+import { sumAlongChain } from '../model/walk';
 import { enabledAlongChain } from '../properties/enabled';
 
 /**
@@ -21,15 +22,7 @@ export interface EntityHandle {
 
 /** World position of a handle: its local position plus those of all ancestors. */
 export function worldPositionOf(handle: EntityHandle): { x: number; y: number; z: number } {
-  const result = { x: handle.x, y: handle.y, z: handle.z };
-  const seen = new Set<EntityHandle>([handle]);
-  for (let p = handle.parent; p && !seen.has(p); p = p.parent) {
-    result.x += p.x;
-    result.y += p.y;
-    result.z += p.z;
-    seen.add(p);
-  }
-  return result;
+  return sumAlongChain(handle, (h) => h.parent, (h) => h);
 }
 
 /** Whether a handle and every ancestor of it are switched on (see `properties/enabled.ts`). */