Add mjModel.tendon_armature (in preparation, not yet implemented)

PiperOrigin-RevId: 743649968
Change-Id: I1d893e3e06afc48b652575bc7837750ab3573670

src/user/user_model.cc

@@ -3390,6 +3390,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     m->tendon_margin[i] = (mjtNum)pte->margin;
     m->tendon_stiffness[i] = (mjtNum)pte->stiffness;
     m->tendon_damping[i] = (mjtNum)pte->damping;
+    m->tendon_armature[i] = (mjtNum)pte->armature;
     m->tendon_frictionloss[i] = (mjtNum)pte->frictionloss;
     m->tendon_lengthspring[2*i] = (mjtNum)pte->springlength[0];
     m->tendon_lengthspring[2*i+1] = (mjtNum)pte->springlength[1];
@@ -4931,6 +4932,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     tendons_[i]->margin = (double)m->tendon_margin[i];
     tendons_[i]->stiffness = (double)m->tendon_stiffness[i];
     tendons_[i]->damping = (double)m->tendon_damping[i];
+    tendons_[i]->armature = (double)m->tendon_armature[i];
     tendons_[i]->frictionloss = (double)m->tendon_frictionloss[i];
 
     if (nuser_tendon) {

src/user/user_objects.cc

@@ -5616,6 +5616,12 @@ void mjCTendon::Compile(void) {
 
     // spatial path
     else {
+      if (armature < 0) {
+        throw mjCError(this,
+                       "tendon '%s' (id = %d): tendon armature cannot be negative",
+                        name.c_str(), id);
+      }
+
       switch (path[i]->type) {
         case mjWRAP_PULLEY:
           // pulley should not follow other pulley
@@ -5657,6 +5663,12 @@ void mjCTendon::Compile(void) {
                            name.c_str(), id, i);
           }
 
+          if (armature > 0) {
+            throw mjCError(this,
+                           "tendon '%s' (id = %d): geom wrapping not supported by tendon armature",
+                           name.c_str(), id);
+          }
+
           // mark geoms as non visual
           model->Geoms()[path[i]->obj->id]->SetNotVisual();
           break;