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/generated.rs

@@ -46,8 +46,8 @@ fn plant(h: &mut Harness, seed: u64, edits: &Edits) -> Tree {
 
 fn resize_one(h: &mut Harness, tree: &Tree, idx: usize, rng: &mut Rng) -> Lens {
     let lens = [
-        Some(Len::abs(20.0 + rng.below(180) as f32)),
-        Some(Len::abs(20.0 + rng.below(180) as f32)),
+        Some(Len::px(20.0 + rng.below(180) as f32)),
+        Some(Len::px(20.0 + rng.below(180) as f32)),
     ];
     let sized = &mut h.rsc[tree.sized[idx]];
     sized.x = lens[0];