Store joint damping and stiffness in the private spec when springdamper is used in the public spec.

PiperOrigin-RevId: 724288204
Change-Id: I7341d5d804e1ea73ded48b8b447addd4e124ab97

src/user/user_model.cc

@@ -1867,6 +1867,10 @@ void mjCModel::AutoSpringDamper(mjModel* m) {
     mjtNum stiffness = inertia / std::max(mjMINVAL, timeconst*timeconst*dampratio*dampratio);
     mjtNum damping = 2 * inertia / std::max(mjMINVAL, timeconst);
 
+    // save stiffness and damping in the private mjsJoints
+    joints_[n]->stiffness = stiffness;
+    joints_[n]->damping = damping;
+
     // assign
     m->jnt_stiffness[n] = stiffness;
     for (int i=0; i<ndim; i++) {