Record what carrying the depth cost and what is left

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

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -409,10 +409,24 @@ today it is O(dirty x depth) per widget settled. It wants agreeing first: every
 place that inserts into or removes from `needs_redraw` has to pair with it, and
 a count that is too low is a stale size rather than a slow frame.
 
-Of the sort's own 24.5%, about 15 points are the depth walk and about 9 are
+`3f7cd82` takes most of it without touching the order: a widget's depth is
-iterating the `HashSet`, which walks by capacity rather than by length. Keeping
+known where it is drawn -- its parent's plus one -- so `Painter` carries it and
-the depth on `ActiveData` would remove the first without changing the order at
+`ActiveData` keeps it, and the choice reads a field instead of walking an
-all, at the price of a second source of truth for it.
+ancestry. Being reused counts as being visited, so the two reuse paths keep it
+current; only a subtree nothing looked at can hold an old one, and nothing
+under an unvisited subtree is being ordered. `depth` asserts the kept value
+against the ancestry in debug builds, and the hundred-seed sweep passes with
+those assertions on, reshuffles included -- those being what moves a widget to
+another parent. Same load at 130 of 260 dirty: 8.16M instructions per frame to
+7.14M, median 0.813 ms to 0.639, and the choosing from 25.8% of the frame to
+4.7%.
+
+What is left of it is iterating the dirty set, which a `HashSet` walks by
+capacity rather than by length. Ordering it -- a `BTreeSet` keyed by the kept
+depth, or a bucket per depth -- would take that too, but `needs_redraw` lives
+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.
 
 **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