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/pad.rs

@@ -12,11 +12,11 @@ impl Widget for Pad {
             .size();
         Size {
             x: Len {
-                abs: inner.x.abs + self.padding.left + self.padding.right,
+                px: inner.x.px + self.padding.left + self.padding.right,
                 ..inner.x
             },
             y: Len {
-                abs: inner.y.abs + self.padding.top + self.padding.bottom,
+                px: inner.y.px + self.padding.top + self.padding.bottom,
                 ..inner.y
             },
         }
@@ -55,10 +55,10 @@ impl Padding {
     }
     pub fn region(&self) -> UiRegion {
         let mut region = UiRegion::FULL;
-        region.x.start.abs += self.left;
-        region.y.start.abs += self.top;
-        region.x.end.abs -= self.right;
-        region.y.end.abs -= self.bottom;
+        region.x.start.px += self.left;
+        region.y.start.px += self.top;
+        region.x.end.px -= self.right;
+        region.y.end.px -= self.bottom;
         region
     }
     pub fn x(amt: impl UiNum) -> Self {