Split the performance section so a fresh reader can find the settle order

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

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -377,6 +377,8 @@ holds a *drawing* keyed on what produced it, and the widget still draws. The
 five reference renders and the resize render are byte-identical, and the
 sweep passes.
 
+### Settling a dirty set, and why the order is not free to change
+
 **Why picking any dirty widget does not work, found 2026-09-14.** `try_reuse`
 asks whether the widget in front of it is dirty and, if not, hands its parent
 the size it last reported. It does not ask whether a dirty widget sits under it
@@ -456,6 +458,8 @@ hits and only two lines differ between the builds. Nothing here rests on that
 difference, but it means those two rows are worth re-measuring before anyone
 builds on them.
 
+### Dirtying many widgets at once
+
 **A frame that dirties many widgets at once was not being checked, and it is
 where the settle order shows.** `77bb75e` adds two generated cases -- every
 declared size changing together, and a spread of widgets marked for redraw