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>

tests/revision_cost.rs

@@ -94,11 +94,7 @@ fn build(h: &mut Harness, rows: usize) -> Vec<WidgetId> {
     let mut col = Span::empty(Dir::DOWN);
     for _ in 0..rows {
         let mut row = Span::empty(Dir::RIGHT);
-        row.push(
-            rect(Color::RED)
-                .width(Len::abs(40.0))
-                .add_strong(&mut h.rsc),
-        );
+        row.push(rect(Color::RED).width(Len::px(40.0)).add_strong(&mut h.rsc));
         let mut body = Span::empty(Dir::DOWN);
         let para = wtext(words(&mut rng, 12, 52))
             .size(16)