Raise warning when attaching a spec with pending keyframes to a different spec.

PiperOrigin-RevId: 685677622
Change-Id: Iba522c523ebd2afe64d6ab28c6b6b4b3cd399cf8

src/user/user_model.h

@@ -297,7 +297,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   void MakeData(const mjModel* m, mjData** dest);
 
   // resolve keyframe references
-  void StoreKeyframes();
+  void StoreKeyframes(mjCModel* dest);
 
   // map from default class name to default class pointer
   std::unordered_map<std::string, mjCDef*> def_map;