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>

src/random.rs

@@ -142,7 +142,7 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
 
     fn len(&mut self) -> Option<Len> {
         match self.rng.below(4) {
-            0 => Some(Len::abs(20.0 + self.rng.below(180) as f32)),
+            0 => Some(Len::px(20.0 + self.rng.below(180) as f32)),
             1 => Some(Len::REST),
             _ => None,
         }