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/replace_cost.rs

@@ -37,7 +37,7 @@ fn replacing_rows_every_frame() {
     }
     h.set_root(span);
     for i in 0..FRAMES {
-        h.rsc[first].y = Some(Len::abs(40.0 + (i % 2) as f32));
+        h.rsc[first].y = Some(Len::px(40.0 + (i % 2) as f32));
         h.frame();
     }
 }