Constrain offered boxes with independent widget size bounds

tests/layout_diagnostics.rs

@@ -264,7 +264,7 @@ fn layout_cost() {
             harness
                 .rsc
                 .widgets_mut()
-                .set_size_rule(sized, Axis::X, SizeRule::Exact(len));
+                .set_size_rule(sized, Axis::X, SizeRule::from(len));
         });
     }