Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

src/user/user_model.cc

@@ -4759,6 +4759,11 @@ bool mjCModel::CopyBack(const mjModel* m) {
     return false;
   }
 
+  if (spec.element->signature != m->signature) {
+    errInfo = mjCError(0, "incompatible signatures in CopyBack");
+    return false;
+  }
+
   // option and visual
   option = m->opt;
   visual = m->vis;