Change mjSpec and mjModel signature mechanism.

The signature now contains the necessary information to safely perform `bind`. The private UIDs are now removed.

Fixes an issue of changing signature when compiling a copy of an mjSpec.

PiperOrigin-RevId: 744670626
Change-Id: Id3c66419cf2afbe78e91bc4b37d2299f5ec00ab1

src/user/user_objects.h

@@ -219,7 +219,6 @@ 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