Copy UIDs during mj_copySpec.

This will produce the same mjSpec signature after a deep copy.

PiperOrigin-RevId: 743538018
Change-Id: Ic0f5b06a00e8883cb92dc98159db3bea02cb7a36

src/user/user_model.cc

@@ -297,7 +297,7 @@ void mjCModel::CopyList(std::vector<T*>& dest,
     // copy the element from the other model to this model
     if (deepcopy_) {
       source[i]->ForgetKeyframes();
-      candidate->uid = GetUid();
+      candidate->uid = source[i]->uid;
     } else {
       candidate->AddRef();
     }