Grow scrolling into the random trees

Scrolling is the one thing in these trees that reads the pixel length of its
box, and the one that hands its child a box longer than its own, so a warm
layout under it has to be rebuilt where the rest can be carried over. A
sixth of the nodes at each level is now a scroll over a subtree, on either
axis.

Four of a hundred seeds now grow nothing but wrappers, so `reshuffled`
returns early where there is no span to shuffle: a case with nothing to do
is not the same as a shuffle that had no effect, which is what the assertion
below it is for.

50 tests, and the ignored sweep over 100 seeds and eight scenarios, 800
comparisons.

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

tests/generated.rs

@@ -188,6 +188,11 @@ fn changed_size(seed: u64) {
 fn reshuffled(seed: u64, shuffle: Shuffle) {
     let mut warm = Harness::new((900, 1200));
     let mut grown = plant(&mut warm, seed, &Edits::default());
+    // Some seeds grow nothing but wrappers, and a shuffle with no span to
+    // shuffle is not the same thing as one that had no effect.
+    if grown.spans.is_empty() {
+        return;
+    }
     let before: Vec<_> = grown.ids.iter().map(|id| warm.region(id)).collect();
 
     let (spans, _held) = reshuffle(&mut warm, &mut grown, shuffle);