Move fullinertia from mjCAlternative to mjCBody with correction to handle URDF with 0 inertias.

PiperOrigin-RevId: 604812870
Change-Id: I018254d1362a654a4f99bf5e621d5abc7129a0c6

src/user/user_model.cc

@@ -20,6 +20,7 @@
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
+#include <iostream>
 #include <map>
 #include <string>
 #include <string_view>
@@ -2551,9 +2552,8 @@ void mjCModel::FuseStatic(void) {
         }
 
         // compute principal axes of inertia
-        mjCAlternative alt;
-        mjuu_copyvec(alt.fullinertia, toti, 6);
-        const char* err1 = alt.Set(par->iquat, par->inertia, degree, euler);
+        mjuu_copyvec(par->fullinertia, toti, 6);
+        const char* err1 = par->FullInertia(par->iquat, par->inertia);
         if (err1) {
           throw mjCError(NULL, "error '%s' in fusing static body inertias", err1);
         }