feat(editor): object save/load JSON, working duplicate, export objects

- Object placer save/load: objects.json persists placed M2/WMO objects
  across sessions (path, position, rotation, scale, type)
- Fixed Duplicate button in Object panel: now actually creates a copy
  with correct path/type/scale instead of being a no-op stub
- Export Zone now saves objects.json alongside ADT/WDT/creatures/manifest
- Object JSON loader parses all fields for full round-trip

tools/editor/editor_app.cpp

@@ -597,6 +597,12 @@ void EditorApp::exportZone(const std::string& outputDir) {
         npcSpawner_.saveToFile(npcPath);
     }
 
+    // Save placed objects
+    if (objectPlacer_.objectCount() > 0) {
+        std::string objPath = base + "/objects.json";
+        objectPlacer_.saveToFile(objPath);
+    }
+
     // Write zone manifest (for client loading)
     ZoneManifest manifest;
     manifest.mapName = loadedMap_;