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_model.h

@@ -324,9 +324,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   // set attached flag
   void SetAttached(bool deepcopy) { attached_ |= !deepcopy; }
 
-  // get new uid
-  int GetUid() { return uid_count_++; }
-
  private:
   // settings for each defaults class
   std::vector<mjCDef*> defaults_;
@@ -441,6 +438,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   void MarkPluginInstance(std::unordered_map<std::string, bool>& instances,
                           const std::vector<T*>& list);
 
+  // print the tree of a body
+  std::string PrintTree(const mjCBody* body, std::string indent = "");
+
   // generate a signature for the model
   uint64_t Signature();
 
@@ -449,7 +449,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   std::vector<mjKeyInfo> key_pending_;  // attached keyframes
   bool deepcopy_;     // copy objects when attaching
   bool attached_ = false;  // true if model is attached to a parent model
-  int uid_count_ = 0;  // unique id count for all objects
   std::unordered_map<const mjsCompiler*, mjSpec*> compiler2spec_;  // map from compiler to spec
 };
 #endif  // MUJOCO_SRC_USER_USER_MODEL_H_