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/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();