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>

src/random.rs

@@ -120,7 +120,7 @@ impl Widget for Branch {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = UiSpan::new(Len::ZERO, cut).shifted_desc();
         let measured = painter
-            .widget_at(&self.probe, top.axis(Axis::Y))
+            .widget_at(&self.probe, top.on_axis(Axis::Y))
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -135,7 +135,7 @@ impl Widget for Branch {
         painter.window_holds(Axis::X, holds.through(len));
 
         let below = UiSpan::new(cut, painter.region_len(Axis::Y)).shifted_desc();
-        let place = below.axis(Axis::Y);
+        let place = below.on_axis(Axis::Y);
         match px > threshold {
             true => painter.widget_at(&self.wide, place),
             false => painter.widget_at(&self.narrow, place),