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/position/span.rs

@@ -24,13 +24,13 @@ impl Widget for Span {
                 Some(len) => len,
                 None => painter.place(child, region).len(axis),
             };
-            cursor.abs += len.abs + self.gap;
+            cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);
         }
 
         let gap = self.gap * self.children.len().saturating_sub(1) as f32;
-        let total = lens.iter().fold(Len::abs(gap), |sum, len| sum + *len);
+        let total = lens.iter().fold(Len::px(gap), |sum, len| sum + *len);
 
         let mut start = UiScalar::rel_min();
         let mut ortho = Len::ZERO;
@@ -38,12 +38,12 @@ impl Widget for Span {
             let mut span = UiSpan::FULL;
             span.start = start;
             if len.rest > 0.0 {
-                let offset = UiScalar::new(total.rel, total.abs);
+                let offset = UiScalar::new(total.rel, total.px);
                 let rel_end = UiScalar::rel(len.rest / total.rest);
                 let end = (UiScalar::rel_max() + start) - offset;
                 start = rel_end.within(&start.to(end));
             }
-            start.abs += len.abs;
+            start.px += len.px;
             start.rel += len.rel;
             span.end = start;
             let mut region = UiRegion::from_axis(axis, span, UiSpan::FULL);
@@ -55,9 +55,9 @@ impl Widget for Span {
             if used.rel > 0.0 || used.rest > 0.0 {
                 ortho = Len::REST;
             } else if ortho.rest == 0.0 {
-                ortho.abs = ortho.abs.max(used.abs);
+                ortho.px = ortho.px.max(used.px);
             }
-            start.abs += self.gap;
+            start.px += self.gap;
         }
 
         let along = match total.rest == 0.0 && total.rel == 0.0 {