Copy attached spec before attaching anything else.

Spec were copied after bodies are attached, so the reference to the source spec got lost during mj_copySpec().

PiperOrigin-RevId: 701287267
Change-Id: I55eff53c7ae9d42b03957be48ea506b80d274e03

src/user/user_model.cc

@@ -182,6 +182,11 @@ mjCModel& mjCModel::operator=(const mjCModel& other) {
     *static_cast<mjCModel_*>(this) = static_cast<const mjCModel_&>(other);
     *static_cast<mjSpec*>(this) = static_cast<const mjSpec&>(other);
 
+    // copy attached specs first so that we can resolve references to them
+    for (const auto* s : other.specs_) {
+      specs_.push_back(mj_copySpec(s));
+    }
+
     // the world copy constructor takes care of copying the tree
     mjCBody* world = new mjCBody(*other.bodies_[0], this);
     bodies_.push_back(world);
@@ -398,9 +403,6 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     }
     CopyList(numerics_, other.numerics_);
     CopyList(texts_, other.texts_);
-    for (const auto* s : other.specs_) {
-      specs_.push_back(mj_copySpec(s));
-    }
   }
   CopyList(flexes_, other.flexes_);
   CopyList(pairs_, other.pairs_);