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/widget/text/mod.rs

@@ -72,7 +72,7 @@ impl TextView {
 
         let tex = self.render(painter);
         let region = tex.size.align(align);
-        let size = Size::abs(tex.size);
+        let size = Size::px(tex.size);
         let within = region.within(&painter.region());
         painter.glyphs(tex, within);
         (region, size)