Add signature to mjSpec and mjModel and use it to perform safe bind to mjModel and mjData.

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

src/user/user_objects.h

@@ -219,6 +219,7 @@ class mjCBoundingVolumeHierarchy : public mjCBoundingVolumeHierarchy_ {
 class mjCBase_ : public mjsElement {
  public:
   int id;                 // object id
+  int uid;                // unique identifier
   std::string name;       // object name
   std::string classname;  // defaults class name
   std::string info;       // error message info set by the user