Say layout's operations by name, and index a pair by its axis

Four rounds over the same idea: an expression that needed a comment to say
what it computed wanted to be a named operation.

The placement description is built by chaining off the value that says it.
`UiSpan::within_desc`/`shifted_desc` and `Len::as_desc` replace the
`PlaceDescAxis::` constructors, `PlaceDescAxis::axis` lifts one axis into a
pair with the whole box across it, and `PlaceDesc::per_axis` covers the case
where the two axes differ. `beside` is dropped: `from_axis` already said it.

Seven module-level functions become methods on the value each took first --
`Widgets::declared_lens`, `LayoutLen::fills`, `PlaceDesc::placement` and
`::rel_base_and_region`, `Size::within_box`, `UiRegion::at_origin` and
`::as_translation`.

`UiSpan::place` is the aligned-placement rule, which was written out three
times; `LayoutLen::without_leftover` is the sibling `apply_leftover` never
had, at six sites; `is_px` and `is_only_leftover` name field comparisons the
surrounding comments had to translate; `Holds::covers` was interval
containment spelled out by hand. A span's `shared` loses the two arguments
that did not vary across its loop.

`LayoutHolds` was four two-element arrays where every other pair here is a
struct of two per-axis values, so nothing it did could be written once.
It becomes `AxisHolds` on `x` and `y`, and `and`, `covers` and `contains`
lose their loops.

Every pair gets `Index<Axis>`/`IndexMut<Axis>` through one macro, and the
eighteen `axis`/`axis_mut` methods go. `const_index` keeps the accessors
usable in const context.

Cold layout is unchanged: `layout_dump` over 400 depth-5 trees is identical
to 58ce74d byte for byte, across all 34,492 boxes.

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

tests/cases/retained.rs

@@ -214,10 +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,
-            PlaceDesc::new(PlaceDescAxis::WHOLE, PlaceDescAxis::shifted(top)),
-        );
+        painter.widget_at(&self.inner, top.shifted_desc().axis(Axis::Y));
         Size::LEFTOVER
     }
 }
@@ -876,10 +873,7 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x),
-                    PlaceDescAxis::shifted(self.region.y),
-                ),
+                PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
             );
             Size::LEFTOVER
         }
@@ -965,8 +959,8 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x).fills(),
-                    PlaceDescAxis::shifted(self.region.y).fills(),
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
                 )
                 .rel_base(Axis::X, self.frame.x.len()),
             );
@@ -1135,8 +1129,8 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(self.region.x).fills(),
-                    PlaceDescAxis::shifted(self.region.y).fills(),
+                    self.region.x.shifted_desc().fills(),
+                    self.region.y.shifted_desc().fills(),
                 ),
             );
             Size::LEFTOVER
@@ -1222,12 +1216,10 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
             painter.widget_at(
                 &self.child,
                 PlaceDesc::new(
-                    PlaceDescAxis::shifted(UiSpan::new(
-                        Len::px(self.start),
-                        Len::px(self.start + 200.0),
-                    ))
-                    .fills(),
-                    PlaceDescAxis::shifted(UiSpan::FULL).fills(),
+                    UiSpan::new(Len::px(self.start), Len::px(self.start + 200.0))
+                        .shifted_desc()
+                        .fills(),
+                    UiSpan::FULL.shifted_desc().fills(),
                 ),
             );
             Size::LEFTOVER
@@ -1263,10 +1255,7 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    PlaceDesc::new(
-                        PlaceDescAxis::shifted(self.region.x),
-                        PlaceDescAxis::shifted(self.region.y),
-                    ),
+                    PlaceDesc::new(self.region.x.shifted_desc(), self.region.y.shifted_desc()),
                 )
                 .size()
         }
@@ -1283,8 +1272,8 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
                     .widget_at(
                         &self.child,
                         PlaceDesc::new(
-                            PlaceDescAxis::shifted(self.region.x).fills(),
-                            PlaceDescAxis::shifted(self.region.y).fills(),
+                            self.region.x.shifted_desc().fills(),
+                            self.region.y.shifted_desc().fills(),
                         ),
                     )
                     .size(),