Fix damper kv inheritance from default classes in XML native reader

Ensured dampers correctly read inherited values from gainprm[2].

PiperOrigin-RevId: 945086959
Change-Id: I10900f3ad057036115da88ad6d649d3d058e7373

src/xml/xml_native_reader.cc

@@ -2561,7 +2561,8 @@ void mjXReader::OneActuator(XMLElement* elem, mjsActuator* actuator) {
 
   // damper
   else if (type == "damper") {
-    double kv = 0;
+    bool inherited = (actuator->gaintype == mjGAIN_AFFINE);
+    double kv = inherited ? -actuator->gainprm[2] : 0;
     ReadAttr(elem, "kv", 1, &kv, text);
     err = mjs_setToDamper(actuator, kv);
   }