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>

src/random.rs

@@ -358,14 +358,17 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
             children,
             dir,
             gap: Px::from_int(self.rng.below(3) as i32 * 4),
-            // Derive this from an existing choice: a seed must keep growing
-            // the same tree when the generator gains another configuration.
-            ortho: match dir.axis {
-                Axis::X => OrthoSize::Full,
-                Axis::Y => OrthoSize::Children,
-            },
         }
         .add(self.rsc);
+        // A row takes the height it is given rather than its tallest child,
+        // which is a rule beside it. Derived from an existing choice and
+        // consuming no randomness: a seed must keep growing the same tree
+        // when the generator gains another configuration.
+        if dir.axis == Axis::X {
+            self.rsc
+                .widgets_mut()
+                .set_size_rules(id, None, Some(Len::rel(1.0)));
+        }
         self.tree.ids.push(id.id());
         self.tree.spans.push(Spanned { id, spares, grown });
         id.add_strong(self.rsc)