Branch on a measurement, so a wrong one shows as a different tree

Comparing boxes catches a widget that moved. It does not catch one that
measured a child, was handed an answer a cold start would not have given,
and took the other branch -- the same defect, arriving where a pixel
comparison cannot see it. Branching on what the painter tells you is
something a widget is allowed to do, so the library owes the same answer
warm and cold; only a widget changing its own configuration is exempt.

`random::Branch` measures a child and draws one of two others on the
result, with both grown either way so the ids match whichever is drawn.
It joins the generator, which makes every existing scenario a control-flow
oracle as well as a geometric one. `tests/determinism.rs` is the same
widget by hand across eight thresholds, including either side of the
answer, and is the fast check -- the sweep is a fuzzer and confirms at the
end rather than being iterated against.

A span behind a branch nobody took is not drawn, so shuffling it cannot
move anything; `reshuffled` now treats that as vacuous, the way it already
treats a tree with no spans, rather than as a shuffle that had no effect.

Both new tests pass, and the sweep passes at depth 4 and 5 over 200 seeds.

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

tests/generated.rs

@@ -224,8 +224,15 @@ 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() {
+    // shuffle is not the same thing as one that had no effect. A span behind
+    // a branch nobody took is the same kind of nothing: it is not drawn, so
+    // shuffling it cannot move anything.
+    let shuffles = grown
+        .spans
+        .iter()
+        .step_by(3)
+        .any(|span| warm.region(&span.id.id()).is_some());
+    if !shuffles {
         return;
     }
     let before: Vec<_> = grown.ids.iter().map(|id| warm.region(id)).collect();