Rename a length's abs component to px

`dp` is coming, and then `abs` says which of the two it is not. The
component has always been a pixel count, so the name that admits it is
the one that leaves room for a second unit beside it.

Mechanical: the field on `Len` and `UiScalar`, their constructors,
`to_abs`/`get_abs`, the matching WGSL struct member and the locals
composing it. Field order and types are unchanged, so the `Pod` layout
the shader reads is the same bytes. `f32::abs` is untouched.

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

core/src/orientation/align.rs

@@ -144,10 +144,10 @@ impl UiScalar {
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
         let mut start = UiScalar::rel(rel);
-        start.abs -= self.abs * rel;
+        start.px -= self.px * rel;
         start.rel -= self.rel * rel;
         let mut end = UiScalar::rel(rel);
-        end.abs += self.abs * (1.0 - rel);
+        end.px += self.px * (1.0 - rel);
         end.rel += self.rel * (1.0 - rel);
         UiSpan { start, end }
     }