Dirty many widgets at once, which nothing was checking

Every generated case changes one thing: four declared sizes, or one
span's children, or the output. A frame settling one dependency path
says nothing about a frame settling a set of them that overlap, which
is the case the settle order exists for.

So two more: every declared size in the tree changing at once, and a
spread of widgets marked for redraw together. The second changes
nothing, which is the point -- no box may move, and the order the
dirty set is taken in is all that can make one. The hundred-seed sweep
is 1,000 comparisons now rather than 800, and passes.

`IRIS_PHASE=many` is the same load for the diagnostics rig, with
`IRIS_DIRTY` widgets marked per frame. It says what one repainting leaf
cannot: at 130 of 260 widgets, choosing which dirty widget to settle
next is 24.5% of the frame, because the dirty set is scanned once per
widget settled and a hash set is walked by capacity rather than by
length. Memoizing the depth walk inside one scan does not pay -- it
trades parent lookups for memo lookups and costs 4% more instructions --
so the fix is to stop rescanning, which changes the order widgets
settle in and wants agreeing first.

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

tests/layout_diagnostics.rs

@@ -11,8 +11,9 @@
 //!       -e cycles:u,instructions:u cargo test --release \
 //!       --test layout_diagnostics -- --ignored --nocapture
 //!
-//! `IRIS_PHASE` is `cold`, `repaint`, `size`, `scroll`, `resize`, or `all`.
-//! `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load.
+//! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
+//! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
+//! `IRIS_DIRTY` how many widgets `many` marks at once.
 
 use iris::harness::Harness;
 use iris::prelude::*;
@@ -165,7 +166,7 @@ fn layout_cost() {
     assert!(frames > 0, "IRIS_FRAMES must be greater than zero");
     let phase = env("IRIS_PHASE", String::from("all"));
     assert!(
-        ["all", "cold", "repaint", "size", "scroll", "resize"].contains(&phase.as_str()),
+        ["all", "cold", "repaint", "many", "size", "scroll", "resize"].contains(&phase.as_str()),
         "unknown IRIS_PHASE {phase:?}"
     );
     let selected = |name| phase == "all" || phase == name;
@@ -194,6 +195,22 @@ fn layout_cost() {
         });
     }
 
+    if selected("many") {
+        let (mut harness, tree) = warm(seed, depth);
+        trace_selected(&tree);
+        // Spread through the tree rather than taken from one subtree, so the
+        // dependency paths the frame settles overlap.
+        let wanted = env("IRIS_DIRTY", 32_usize).max(1);
+        let step = (tree.ids.len() / wanted).max(1);
+        let dirty: Vec<_> = tree.ids.iter().copied().step_by(step).collect();
+        println!("marking {} of {} widgets", dirty.len(), tree.ids.len());
+        run("many", frames, &mut harness, move |harness, _| {
+            for &id in &dirty {
+                harness.rsc.widgets_mut().get_dyn_mut(id);
+            }
+        });
+    }
+
     if selected("size") {
         let (mut harness, tree) = warm(seed, depth);
         trace_selected(&tree);