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/ui/render_state.rs

@@ -325,7 +325,7 @@ impl UiRenderState {
         self.moves
             .resolve(slot, region)
             .size()
-            .to_abs(self.output_size)
+            .to_px(self.output_size)
     }
 
     /// A clean widget's retained size, when the offered pixel axes which