Record that dropping the settle order costs more than keeping it

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

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -428,6 +428,30 @@ on `Widgets` and is inserted from places with no view of the tree, so either
 means a second structure inside the render state kept in step with it. For 4.7%
 that is not obviously worth the coupling.
 
+**Going order-free costs more than the order now does.** Measured 2026-09-14
+with the depth carried, instructions per frame, on the same three loads:
+
+| | 32 dirty | 130 dirty | resize |
+| --- | --- | --- | --- |
+| ordered, no check (what is in) | 2.80M | 7.14M | 6.70M |
+| ordered, with the check | 2.83M | 7.18M | 6.72M |
+| arbitrary, with the check | 3.12M | 7.38M | 6.28M |
+
+So the check is nearly free where the order already makes it redundant, and
+dropping the order costs 11% on a small dirty set. **Keeping the answer does
+not help: it never hits.** The dirty set moves about once per check -- every
+widget drawn takes its own mark out -- so an answer worked out against one
+state of it is asked for against another, and a generation stamp invalidates
+everything each time. Only a count maintained up the reader chain as marks come
+and go would answer in O(1) without an invalidation to lose, and that is the
+version still unbuilt.
+
+One thing not understood: the arbitrary-order rows above are consistently ~6%
+cheaper with the kept answer in than without it, though the kept answer never
+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.
+
 **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