Delete OrthoSize, and run the seeds in parallel

A span is as long across itself as its longest child, unless a rule beside it
already says how long it is -- and then reading the children answers nothing
and only makes its size depend on theirs. `OrthoSize::Full` was that second
case written twice, once as an enum on the span and once as the rule that
actually decides; `Painter::ruled` lets the span ask which it is in. The
widget under a rule still does not learn what the rule says, only that its
answer for that axis is not wanted.

The fuzzers grow, lay out and drop a tree within one seed, so the seeds share
nothing and take a thread each, one short of every core. Measured here: the
generated oracle's hundred seeds went from 68 s to 10 s, and a shrinker case
at 300 seeds from 18 s to 3.5 s. A seed that fails still shrinks and panics
on its own thread, and `std::thread::scope` carries that out.

The shrinker now allows the two steps the oracle already did -- the deeper
trees these grow reach a second composition, and a step is a thousandth of a
pixel.

Checked: fmt, clippy, 102 tests, all five shrinker cases at 300 seeds, 100
generated seeds, and five examples byte-identical at 1920x1200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/layout.rs

@@ -21,27 +21,21 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
 }
 
 #[test]
-fn a_full_ortho_span_reports_relative_full() {
+fn a_span_ruled_across_itself_does_not_measure_its_children_there() {
     let mut h = Harness::new((400, 200));
     let child = rect(Color::RED).height(40).add(&mut h.rsc);
-    let span = (child,)
-        .span(Dir::RIGHT)
-        .ortho(OrthoSize::Full)
-        .add(&mut h.rsc);
+    let span = (child,).span(Dir::RIGHT).height(rel(1.0)).add(&mut h.rsc);
     h.set_root(span);
 
     assert_eq!(h.render.active[&span.id()].size.y, Len::rel(1.0));
 }
 
 #[test]
-fn a_children_ortho_span_reports_its_tallest_fixed_child() {
+fn a_span_reports_its_tallest_fixed_child() {
     let mut h = Harness::new((400, 200));
     let short = rect(Color::RED).height(40).add(&mut h.rsc);
     let tall = rect(Color::BLUE).height(70).add(&mut h.rsc);
-    let span = (short, tall)
-        .span(Dir::RIGHT)
-        .ortho(OrthoSize::Children)
-        .add(&mut h.rsc);
+    let span = (short, tall).span(Dir::RIGHT).add(&mut h.rsc);
     h.set_root(span);
 
     assert_eq!(h.render.active[&span.id()].size.y, Len::px(70.0));
@@ -211,10 +205,7 @@ fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
     let filler = rect(Color::GREEN).add(&mut h.rsc);
     // This column is an item in a row, so it takes the width left for it
     // rather than asking for a full row-width in addition to the bar.
-    let column = (row, filler)
-        .span(Dir::DOWN)
-        .ortho(OrthoSize::Children)
-        .add(&mut h.rsc);
+    let column = (row, filler).span(Dir::DOWN).add(&mut h.rsc);
     let bar = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((bar, column).span(Dir::RIGHT));
     assert_corners!(h, inner, (110, 10), (390, 30));

tests/cases/retained.rs

@@ -300,13 +300,10 @@ fn a_resize_does_not_redraw_what_the_shader_can_move() {
 }
 
 #[test]
-fn a_full_ortho_span_moves_its_child_without_redrawing_it() {
+fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
     let mut h = Harness::new((400, 200));
     let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
-    let span = (leaf,)
-        .span(Dir::RIGHT)
-        .ortho(OrthoSize::Full)
-        .add(&mut h.rsc);
+    let span = (leaf,).span(Dir::RIGHT).height(rel(1.0)).add(&mut h.rsc);
     h.set_root(span);
     let settled = draws.get();
 

tests/cases/unsettled.rs

@@ -160,7 +160,6 @@ fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span
         children,
         dir: Dir::RIGHT,
         gap: Px::ZERO,
-        ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
     let span_handle = span;
@@ -214,7 +213,6 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         children: inner_children,
         dir: Dir::RIGHT,
         gap: Px::ZERO,
-        ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
     let block = rect(Color::RED).add(&mut h.rsc);
@@ -228,7 +226,6 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         children: outer_children,
         dir: Dir::RIGHT,
         gap: Px::ZERO,
-        ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
     // Carried no rule even before rules were a property: it is here to be a
@@ -337,7 +334,6 @@ fn plant_boundary(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         children: pair,
         dir: Dir::DOWN,
         gap: Px::ZERO,
-        ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
     // Takes the whole box on its own, so the span above has nothing left to
@@ -357,7 +353,6 @@ fn plant_boundary(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         children: inner_children,
         dir: Dir::DOWN,
         gap: Px::ZERO,
-        ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
     h.rsc