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/render/shader/rect.wgsl

@@ -34,6 +34,6 @@ fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner:
     // vec from center to pixel
     let p = pixel_pos - rect_center;
     // vec from inner rect corner to pixel
-    let q = abs(p) - (rect_corner - radius);
+    let q = px(p) - (rect_corner - radius);
     return length(max(q, vec2(0.0))) - radius;
 }