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/editor/state.ts

@@ -136,8 +136,28 @@ export class EditorState {
    * In play mode the command changes the runtime copy only (see `enterPlay`).
    */
   execute(command: Command): boolean {
-    try {
+    return this.runEdit(() => {
       this.commands.execute(command);
+      return true;
+    });
+  }
+
+  undo(): void {
+    this.runEdit(() => this.commands.undo());
+  }
+
+  redo(): void {
+    this.runEdit(() => this.commands.redo());
+  }
+
+  /**
+   * Runs one history operation. A `CommandError` is the documented way for a command to
+   * reject an edit, so it becomes a notice; anything else is a real defect and keeps
+   * travelling. Undo and redo go through here too – they can reject just like `execute`.
+   */
+  private runEdit(operation: () => boolean): boolean {
+    try {
+      if (!operation()) return false;
     } catch (error) {
       if (error instanceof CommandError) {
         this.setNotice(error.message);
@@ -149,16 +169,6 @@ export class EditorState {
     return true;
   }
 
-  undo(): void {
-    if (!this.commands.undo()) return;
-    this.markDirty();
-  }
-
-  redo(): void {
-    if (!this.commands.redo()) return;
-    this.markDirty();
-  }
-
   /**
    * The model was mutated directly (live preview while scrubbing). Notifies listeners
    * but does not touch the undo stack or the save status – the committing command does.
@@ -246,7 +256,8 @@ export class EditorState {
   }
 }
 
-function sameSelection(a: Selection, b: Selection): boolean {
+/** Whether two selections mean the same thing (same kind, same ids, same order). */
+export function sameSelection(a: Selection, b: Selection): boolean {
   if (a === b) return true;
   if (!a || !b || a.kind !== b.kind) return false;
   if (a.kind === 'asset' && b.kind === 'asset') return a.assetId === b.assetId;