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/widget/position/span.rs

@@ -33,7 +33,7 @@ impl Widget for Span {
                     // Across itself the child sits where its own alignment
                     // says, in the whole of the row: a span is what contains
                     // its children there, and nothing divides that axis.
-                    let room = along(cursor, far).shifted_desc().axis(axis);
+                    let room = along(cursor, far).shifted_desc().on_axis(axis);
                     painter.widget_at(child, room).len(axis)
                 }
             };
@@ -119,7 +119,7 @@ impl Widget for Span {
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
             let slot = along(from, start);
-            let mut place = slot.shifted_desc().fills().axis(axis);
+            let mut place = slot.shifted_desc().fills().on_axis(axis);
             if len.leftover > Weight::ZERO && shares {
                 place = place.rel_base(axis, slot.len());
             }