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>

core/src/ui/painter.rs

@@ -260,9 +260,9 @@ impl<'a> Painter<'a> {
             let mut region = origin;
             region.x.end = region.x.start;
             region.y.end = region.y.start;
-            let mut region = region.offset(UiVec2::abs(glyph.offset));
-            region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
-            region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
+            let mut region = region.offset(UiVec2::px(glyph.offset));
+            region.x.end = region.x.start + UiScalar::px(glyph.entry.width as f32);
+            region.y.end = region.y.start + UiScalar::px(glyph.entry.height as f32);
             self.write(
                 kind,
                 GlyphPrimitive {
@@ -306,7 +306,7 @@ impl<'a> Painter<'a> {
         self.reads_output = [true; 2];
         self.size_box_inputs = [true; 2];
         let region = self.state.moves.resolve(self.move_idx, self.region);
-        region.size().to_abs(self.state.output_size)
+        region.size().to_px(self.state.output_size)
     }
 
     /// One axis of this widget's box in pixels. Prefer this to
@@ -318,7 +318,7 @@ impl<'a> Painter<'a> {
         region
             .size()
             .axis(axis)
-            .to_abs(self.state.output_size.axis(axis))
+            .to_px(self.state.output_size.axis(axis))
     }
 
     pub fn text_data(&mut self) -> &mut TextData {