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/SetBagValueCommand.ts

@@ -0,0 +1,44 @@
+import { type PropertyBag, type PropertyValue, setBagValue } from '../model/Properties';
+import type { Project } from '../model/Project';
+import type { Command } from './Command';
+
+/**
+ * The skeleton every "write one key into a property bag" edit shares: remember what the bag
+ * held, write the new value, put the old one back on undo. A key that is not in the bag is
+ * `undefined` on both ends – as a `computeNext` result it removes the key (this is how
+ * clearing an override works), and as remembered value it makes undo remove it again.
+ *
+ * Subclasses look their target up on every call instead of holding on to it, so an undo whose
+ * target is gone fails with a `CommandError` instead of writing into an orphaned object.
+ */
+export abstract class SetBagValueCommand<T> implements Command {
+  readonly label: string;
+  private previous: PropertyValue | undefined;
+
+  protected constructor(protected readonly key: string, label: string) {
+    this.label = label;
+  }
+
+  /** Throws a `CommandError` when the target does not exist. */
+  protected abstract resolveTarget(project: Project): T;
+
+  protected abstract bagOf(target: T): PropertyBag;
+
+  /**
+   * The value to write, `undefined` to remove the key. Called before anything is mutated, so
+   * this is where a subclass rejects an invalid edit (an unknown property key, say).
+   */
+  protected abstract computeNext(target: T, project: Project): PropertyValue | undefined;
+
+  execute(project: Project): void {
+    const target = this.resolveTarget(project);
+    const bag = this.bagOf(target);
+    const next = this.computeNext(target, project);
+    this.previous = bag[this.key];
+    setBagValue(bag, this.key, next);
+  }
+
+  undo(project: Project): void {
+    setBagValue(this.bagOf(this.resolveTarget(project)), this.key, this.previous);
+  }
+}