Do not delete attached specs after compilation.

Also, copy them when copying the spec.

PiperOrigin-RevId: 688998464
Change-Id: I7229fd8032eafede582194ccf73ab4160fe06768

src/user/user_model.cc

@@ -388,6 +388,9 @@ 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_);
@@ -3648,12 +3651,6 @@ mjModel* mjCModel::Compile(const mjVFS* vfs, mjModel** m) {
     return nullptr;
   }
 
-  // destroy attached specs
-  for (auto spec : specs_) {
-    mj_deleteSpec(spec);
-  }
-  specs_.clear();
-
   // restore error handler, mark as compiled, return mjModel
   _mjPRIVATE__set_tls_error_fn(save_error);
   _mjPRIVATE__set_tls_warning_fn(save_warning);