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/xml/xml_native_writer.cc

@@ -456,7 +456,9 @@ void mjXWriter::OneEquality(XMLElement* elem, mjCEquality* peq, mjCDef* def) {
     case mjEQ_WELD:
       WriteAttrTxt(elem, "body1", peq->name1);
       WriteAttrTxt(elem, "body2", peq->name2);
-      WriteAttr(elem, "relpose", 7, peq->data);
+      WriteAttr(elem, "anchor", 3, peq->data);
+      WriteAttr(elem, "tfratio", 1, peq->data+10);
+      WriteAttr(elem, "relpose", 7, peq->data+3);
       break;
 
     case mjEQ_JOINT: