Drop three things nothing reads, and say what a span makes scalable

`Axis::pair` and `RegionAlign::NEAR` arrived on this branch with no caller
and never got one. `Holds::contains` took `&self` where its five siblings on
the same `Copy` pair of pixels take `self`.

The comment beside a span's cross-axis accumulator said a scalable child
"makes Children scalable too"; `Children` names nothing here, and what it
makes scalable is the span.

core/src/orientation/align.rs

@@ -84,14 +84,6 @@ pub struct RegionAlign {
     pub y: AxisAlign,
 }
 
-impl RegionAlign {
-    /// Both axes at the near edge: the start of a box in its own orientation.
-    pub const NEAR: Self = Self {
-        x: AxisAlign::NEG,
-        y: AxisAlign::NEG,
-    };
-}
-
 impl RegionAlign {
     pub const TOP_LEFT: Self = Self::new(AxisAlign::NEG, AxisAlign::NEG);
     pub const TOP_CENTER: Self = Self::new(AxisAlign::CENTER, AxisAlign::NEG);