Write sensor reftype and refname (if present) to XML file.

PiperOrigin-RevId: 453926332
Change-Id: I61f4aa2dc6186149b22af71eb8bb0fb2ffd84209

src/xml/xml_native_writer.cc

@@ -1559,6 +1559,12 @@ void mjXWriter::Sensor(XMLElement* root) {
     WriteAttr(elem, "cutoff", 1, &psen->cutoff, &zero);
     WriteAttr(elem, "noise", 1, &psen->noise, &zero);
     WriteVector(elem, "user", psen->userdata);
+
+    // add reference if present
+    if (psen->reftype > 0) {
+      WriteAttrTxt(elem, "reftype", mju_type2Str(psen->reftype));
+      WriteAttrTxt(elem, "refname", psen->refname);
+    }
   }
 
   // remove section if empty

test/testdata/model.xml

@@ -121,7 +121,7 @@
     <general joint="wheel_1" biastype="affine" dyntype="integrator" dynprm="1" biasprm="0 -1"/>
   </actuator>
   <sensor>
-    <framepos objtype="site" objname="wheel_0"/>
+    <framepos objtype="site" objname="wheel_0" reftype="site" refname="wheel_2"/>
     <rangefinder site="rf"/>
     <gyro site="wheel_2"/>
     <touch site="wheel_1"/>