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>

tests/layout.rs

@@ -54,7 +54,7 @@ fn a_child_drawn_twice_moves_once() {
     h.set_root((left, centered).span(Dir::RIGHT));
     assert_corners!(h, inner, (100, 0), (300, 200));
 
-    h.rsc[left].x = Some(Len::abs(150));
+    h.rsc[left].x = Some(Len::px(150));
     h.frame();
 
     assert_corners!(h, inner, (150, 0), (350, 200));
@@ -104,7 +104,7 @@ fn a_fixed_box_is_drawn_again_rather_than_stretched() {
     h.set_root(stack.align(Align::TOP));
     assert_corners!(h, panel, (0, 0), (400, 100));
 
-    h.rsc[leaf].y = Some(Len::abs(250));
+    h.rsc[leaf].y = Some(Len::px(250));
     h.frame();
 
     assert_corners!(h, panel, (0, 0), (400, 250));
@@ -119,7 +119,7 @@ fn a_moved_subtree_takes_its_children_with_it() {
     h.set_root((first, row).span(Dir::DOWN));
     assert_corners!(h, inner, (10, 50), (390, 70));
 
-    h.rsc[first].y = Some(Len::abs(80));
+    h.rsc[first].y = Some(Len::px(80));
     h.frame();
 
     // The row is the same shape somewhere else, so one slot moved it and
@@ -140,7 +140,7 @@ fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
     assert_corners!(h, fixed, (100, 0), (150, 200));
     assert_corners!(h, rest, (150, 0), (400, 200));
 
-    h.rsc[bar].x = Some(Len::abs(200));
+    h.rsc[bar].x = Some(Len::px(200));
     h.frame();
 
     // The panel's box is 100 shorter, so the fixed child is the same 50 wide
@@ -163,7 +163,7 @@ fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
     h.set_root((bar, column).span(Dir::RIGHT));
     assert_corners!(h, inner, (110, 10), (390, 30));
 
-    h.rsc[bar].x = Some(Len::abs(200));
+    h.rsc[bar].x = Some(Len::px(200));
     h.frame();
 
     assert_corners!(h, inner, (210, 10), (390, 30));