Say on_axis for the lift, so it is not indexing's word

`PlaceDescAxis::axis(axis)` shared its name with `PlaceDesc`'s extraction,
which is now `Index<Axis>` and reads `place[axis]`. The two go opposite
directions, so they get different words: `on_axis` pairs with the
`from_axis` it is the shorthand for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/retained.rs

@@ -214,7 +214,7 @@ impl Widget for FromHint {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
-        painter.widget_at(&self.inner, top.shifted_desc().axis(Axis::Y));
+        painter.widget_at(&self.inner, top.shifted_desc().on_axis(Axis::Y));
         Size::LEFTOVER
     }
 }