Add two new attributes to weld constraints:

- `anchor` determines the point of wrench application, in the frame of body2.
- `tfratio` scales applied torques relative to applied forces.
- Add visualisation of both anchor points to both weld and connect constraints.
- Add a test model showing how the new weld parameters behave.

PiperOrigin-RevId: 469190483
Change-Id: I20f6da85b09cd2c4c0b5d29eb8003eabb524e55e

src/user/user_objects.cc

@@ -2884,6 +2884,7 @@ mjCEquality::mjCEquality(mjCModel* _model, mjCDef* _def) {
 
   mjuu_zerovec(data, mjNEQDATA);
   data[1] = 1;
+  data[10] = 1;  // torque:force ratio
 
   // clear internal variables
   obj1id = obj2id = -1;
@@ -2971,6 +2972,11 @@ void mjCEquality::Compile(void) {
   if (type==mjEQ_CONNECT) {
     ((mjCBody*)px1)->MakeLocal(anchor, qdummy, data, qunit);
     mjuu_copyvec(data, anchor, 3);
+  } else if (type==mjEQ_WELD) {
+    if (px2) {
+      ((mjCBody*)px2)->MakeLocal(anchor, qdummy, data, qunit);
+      mjuu_copyvec(data, anchor, 3);
+    }
   }
 }