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/chain_cost.rs

@@ -80,7 +80,7 @@ fn fill(ui: &mut UiData, render: &mut UiRenderState, depth: usize) {
         slot = render.moves.push(slot, UiRegion::FULL);
     }
 
-    let px = |v: f32| UiScalar { rel: 0.0, abs: v };
+    let px = |v: f32| UiScalar { rel: 0.0, px: v };
     for i in 0..INSTANCES {
         let x = (i % (SIZE as usize / 2)) as f32 * 2.0;
         let y = (i / (SIZE as usize / 2)) as f32;