Torus: correct the cross_section_normal method -- again

i should add integration tests

crates/coremem/src/geom/region/primitives.rs

@@ -107,7 +107,7 @@ impl HasCrossSection for Torus {
         let axis = self.axis();
         let to_coord = *coord - *self.center();
         // this creates a normal which always points "counter-clockwise" along the shape
-        axis.cross(to_coord)
+        axis.cross(to_coord).with_mag(self.cross_section()).unwrap_or_default()
     }
 }