The first item on the plan landed as two commits on #18. The re-parenting
half turned out to be two defects rather than the predicted one -- the old
parent's child list erasing a subtree it no longer draws, and the settling
walk ordering by a stale depth -- and the `depth()` assertion the note
expected to catch it catches only the second.
`dirty_size_under` is deleted, so the section that argued for it is marked
superseded rather than removed: it says what the bottom-up walk is buying.
iris-ai
3084491b9b