feat(extract): build containment tree and traverse all containers

Add buildTree to mirror how TTS nests objects, with a display label
(Nickname or class Name). Generalize traverseMod to descend into any
object with ContainedObjects, not just bags, and add Nickname to
TTSObject.

packages/extract/src/objects.ts

@@ -1,6 +1,33 @@
 import type { TTSMod, TTSObject } from '@tts/shared';
 import { markParent, traverseMod } from './traverse.js';
 
+/** A node in the containment tree of a save. */
+export interface ObjectTreeNode {
+  object: TTSObject;
+  /** Display label: `Nickname` when present, else the class `Name`. */
+  label: string;
+  children: ObjectTreeNode[];
+}
+
+/**
+ * Build the containment tree of a save, mirroring how TTS nests objects.
+ * Each node carries its object plus a display label and its children.
+ */
+export function buildTree(mod: TTSMod): ObjectTreeNode[] {
+  for (const state of mod.ObjectStates) {
+    markParent(state);
+  }
+  return mod.ObjectStates.map(nodeFrom);
+}
+
+function nodeFrom(o: TTSObject): ObjectTreeNode {
+  return {
+    object: o,
+    label: o.Nickname || o.Name,
+    children: (o.ContainedObjects ?? []).map(nodeFrom),
+  };
+}
+
 /**
  * Flatten all objects in a save into an array.
  * Ensures parent links are set before returning.