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/scroll.rs

@@ -12,7 +12,7 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let output_len = painter.output_len(self.axis);
-        let container_len = UiScalar::abs(painter.px_len(self.axis));
+        let container_len = UiScalar::px(painter.px_len(self.axis));
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then place it at the scrolled offset.
         let known_len = painter.known_len(&self.inner, self.axis, UiRegion::FULL);
@@ -22,8 +22,8 @@ impl Widget for Scroll {
             .unwrap_or_else(|| child.unwrap().axis(self.axis))
             .apply_rest()
             .within_len(container_len)
-            .to_abs(output_len);
-        self.container_len = container_len.to_abs(output_len);
+            .to_px(output_len);
+        self.container_len = container_len.to_px(output_len);
         self.content_len = content_len;
 
         if self.snap_end {