Record the resize walk and the re-parenting defects, and what verified them

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superseded rather than removed: it says what the bottom-up walk is buying.

docs/HANDOFF.md

@@ -10,7 +10,7 @@ stayed.
 
 Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18
 `split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`, head
-**`25e456e`**, pushed. It holds LAYOUT.md §2's position chain, `leftover`,
+**`e44dea3`**, pushed. It holds LAYOUT.md §2's position chain, `leftover`,
 the `Holds` retained-layout contract, region nodes, built-in alignment and
 size rules, fixed-point layout, a box in pixels threaded down the draw, and
 a report read as a fraction of the containing widget. No PR review was
@@ -132,7 +132,13 @@ not exceed one, so `Span`'s `fixed <= 0` branches were only reachable
 through declared fractions; they are ordinary now, and with them boxes of
 negative length passed down to children.
 
-### An answer is not an answer while anything under it is dirty (`0e0d4af`)
+### An answer is not an answer while anything under it is dirty (`0e0d4af`, superseded by `a0693ac`)
+
+**Superseded: `dirty_size_under` is deleted.** Once a resize goes through
+the settling walk the ordering makes the whole category unreachable rather
+than checked, which is Bryan's steer and the better answer. The defect and
+its reasoning are kept below because they say what the ordering is buying.
+
 
 `draw_inner` took an answer from `try_reuse`, which checks only whether the
 widget itself is marked, where `retained_answer` beside it also refused one
@@ -144,17 +150,13 @@ its parent's own draw never goes through it. The placing ask redraws the
 subtree, the descendant's mark is cleared there, and the parent keeps a
 number the tree no longer agrees with.
 
-So `dirty_size_under` is not the optimization its comment claimed. It is
+So `dirty_size_under` was not the optimization its comment claimed: it was
-what makes an answer an answer, and it is asked once in `draw_inner` for
+what made an answer an answer, until the walk made the state it guarded
-both retained routes. Twenty-five rig counters are unchanged on `cold`,
+against impossible to be in.
-`repaint`, `scroll`, `resize` and `size`; `many` makes 18 fewer reuse
-attempts, 17 of which already reported "dirty".
 
 Found at seed 564, depth 6, `shuffle-every-other`, reachable only once a
-span could overflow itself. **The fast test for it is still owed**: the
+span could overflow itself. No hand-built tree ever reproduced it, and the
-divergence needs a reader drawing while a size dependency two levels under
+seeds at depth 4 above fail for some other reason.
-it is dirty, which no hand-built tree has reproduced yet, and the seeds at
-depth 4 above fail for some other reason.
 
 ### A text is handed back a box its own line fits in (landed, `4bd8607`)
 
@@ -216,78 +218,51 @@ loop {
 Bryan's, 2026-09-17, and the right answer where `0e0d4af` was a check:
 "then that entire category of issue can't even occur".
 
-**The walk is sound on its own, and the guard is only there for a second
+**The walk is sound on its own, and the second entry point is closed**
-entry point** (read on 2026-09-17, Bryan asking for either a breaking case
+(`a0693ac`). By induction on depth: when a widget at depth d draws fresh,
-or a proof). By induction on depth: when a widget at depth d draws fresh,
 every dirty widget deeper has been popped, so each is settled or deferred,
 and a deferred one has marked its parent. A clean child asked by that draw
 therefore has a clean subtree, because anything dirty under it would have a
-dirty parent, and so on up to the child itself. `dirty_size_under` cannot
+dirty parent, and so on up to the child itself.
-fire inside `redraw_updates`, which is what dropping it from the suite,
-the shrinker at 400 seeds of depth 5, the oracle at 1000 of depth 6 and
-2000 seeds at depth 4 measured.
 
-The entry point it guards is `update` drawing the root for a resize before
+The entry point that was left was `update` drawing the root for a resize
-the walk runs, top-down over a tree with dirty widgets still in it. Most
+before the walk ran, top-down over a tree with dirty widgets still in it.
-routes through that are safe anyway: `retained_answer` succeeding at the
+It is closed by marking the root instead, so layout is one walk a frame and
-offer implies `try_reuse` succeeds at the placed box, since the settled
+`dirty_size_under` is gone at both call sites. **The root is marked only
-holds are cut by the drawing's holds taken through the placing length, so
+where the new output falls outside what its answer holds for**: that range
-a stale answer is reused
+is the intersection of everything under it, so admitting the new output
-whole and the dirty descendant later reaches its readers through
+says the whole tree stands, and nothing above the root moved. Marking it
-`redraw`'s comparison. The route that is not safe is the one asymmetry
+unconditionally cost the root its own `Holds` -- a leaf root that scales
-between the two: `try_reuse` refuses a drawing on another layer and
+with its box was drawn again on every resize, which two tests caught.
-`retained_answer` does not. A resize frame in which a structural edit
-shifted a clean child's layer, with a dirty descendant under that child
-whose answer changes, hands back the old answer, redraws the subtree fresh
-in `place`, clears the descendant's mark there, and tells nobody.
 
-**Close the entry point rather than test the contrived case.** A resize
+The rig says the cost is scheduling only: on `resize`, one queue pop, one
-marks the root and nothing else, so layout has one walk and the induction
+local redraw and one depth read appear and one failed reuse attempt goes,
-covers everything:
+and every other counter on `cold`, `repaint`, `many`, `size`, `scroll` and
+`resize` is identical.
 
-```rust
+### A subtree that changes hands is recorded on both sides (landed, `e44dea3`)
-pub fn resize(&mut self, size: impl Into<Vec2>, widgets: &mut Widgets) {
-    let size = PxVec2::from_f32(size.into());
-    if size == self.output_size {
-        return;
-    }
-    self.output_size = size;
-    if let Some(root) = self.old_root {
-        widgets.needs_redraw.insert(root);
-    }
-}
-```
 
-`redraw` already asks a parentless widget again in `root_region` against
+A subtree can be reused whole under a different parent -- same box, same
-the output. Then `dirty_size_under` goes at both call sites, `draw_inner`
+layer, same region node, clean -- and nothing in the drawing says it moved.
-and `retained_size`. Bryan's question of whether a resize could instead
+Two things read who its parent is, and both were wrong after one of these.
-mark "the important widgets" up front answers itself: `needs_redraw` is the
-mark, and `Holds` is the exact, per-widget, lazy computation of which
-widgets a new output invalidates; nothing has to be worked out ahead of the
-walk. What it costs: the root always runs its own `draw` on a resize where
-today the whole tree can remap in one `try_reuse`, and a widget dirty in
-the same frame as a resize may draw twice if the root's layout then gives
-it a different box. Drawn widgets should otherwise be identical on the rig;
-check the counters.
 
-**Two facts the induction relies on are not pinned, and both are the shape
+- The **old parent still listed it**, and a parent's next draw undraws
-of `0e0d4af`: state that is right only because something else set it up.**
+  whatever is missing from that list. Two spans under one root, with the
+  root swapping which of them it holds, drew the subtree under the new span
+  and then erased it when the old one drew.
+- Its **depth** was the one it had under the old parent, which is what the
+  settling walk orders by, so a change made under it afterwards settled at
+  the wrong point in the frame.
 
-- `try_reuse` never writes `active.parent`, and neither does the tail of
+Both are written where `draw_inner` already records what the ask decided:
-  `draw_inner`. A subtree reused under a different parent inside the same
+`active.parent` is replaced and the old parent's `children` repaired, and
-  region node keeps the old parent, so a later deferral marks the wrong
+`try_reuse` re-walks the subtree's depths -- only where the top of it
-  widget. The fuzzer never re-parents: `reshuffle` only trades children
+moved, which is what makes that free in the ordinary case. Pinned by
-  between a span and its own spares. A hand-built test moving a child
+`retained::a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left`
-  between two spans should miss a redraw or trip the `depth()` assertion.
+and `..._settles_at_the_depth_it_moved_to`; each fails without one half.
-- `remap_subtree` never updates descendants' `depth`. The debug assertion
-  in `depth()` catches it only for a widget that later goes dirty.
 
-Both fixes are one line where `draw_inner` already writes `given` and
+The fuzzer never re-parents (`reshuffle` only trades children between a
-`answer`, plus a walk in `remap_subtree` or a depth kept relative to the
+span and its own spares), which is why nothing generated reached either.
-nearest region node.
-
-Drawn widgets, widget draws and primitive writes are unchanged on every rig
-phase; `many` pays 51 queue pops for 27 and 1059 depth reads for 410.
 
 ### A span's leftover boundary is its own inverse (landed, `53b00c6`)
 
@@ -799,34 +774,32 @@ above for what closing the rest would cost.
 
 ## Verification at the current head
 
-At `25e456e`:
+At `e44dea3`:
 
 - `cargo fmt --all --check`, `cargo clippy --workspace --all-targets --
-  -D warnings`, `cargo test --workspace`: green, 90 suite tests, 19 core
+  -D warnings`, `cargo test --workspace`: green, 92 suite tests, 19 core
   unit tests, 11 generated cases. Only the long runs and the profiling rigs
   are ignored; no known defect is.
-- The release oracle at 100 seeds in 14.3 s, and **120 seeds in debug** in
+- The release oracle at 100 seeds in 14.2 s, and **120 seeds in debug** in
   59 s -- the debug run exercises the `Holds` assertion in `draw_at`.
-- All fifteen shrinker cases at 400 seeds of depth 5 in 56 s, the oracle at
+- All fifteen shrinker cases at 400 seeds of depth 5 in 57 s, the oracle at
-  1000 seeds of depth 6 in 142 s, and **2000 seeds at depth 4 over all
+  1000 seeds of depth 6 in 143 s, and **2000 seeds at depth 4 over all
-  fifteen cases** in 262 s. The last is not routine and should be: it is
+  fifteen cases** in 260 s. The last is not routine and should be: it is
   the only run that has ever found anything past seed 400.
-- `view`, `minimal`, `random` and `tabs` byte-identical at 1920x1200
+- `view`, `minimal`, `random`, `tabs` and `text` byte-identical at
-  against `ea6dbae`. `random` live-resized from 1920x1200 to 1280x800 is
+  1920x1200 against `25e456e`, as is `tabs` under the recorded replay.
-  byte-identical to a cold 1280x800 render. **`text` is a new picture**:
+  `random` live-resized from 1920x1200 to 1280x800 is byte-identical to a
-  `4bd8607` moved its lower paragraph one pixel, the box being a step wider
+  cold 1280x800 render. Rendered through Venus on the host's RX 7900 XT,
-  and its left edge crossing the shader's snap, and `c8beca5` rewrote the
+  confirmed against `vulkaninfo --summary` in the same session -- an
-  alignment panel, which had all three labels in the middle of a box the
+  llvmpipe fallback makes the same PNG and nothing in it says so.
-  width of the widest of them.
+- All rig counters identical on `cold`, `repaint`, `many`, `size` and
-- Twenty-five rig work counters identical on `cold`, `repaint`, `scroll`,
+  `scroll` across `25e456e`. `resize` gains one queue pop, one local
-  `resize` and `size` across `0e0d4af`; `many` differs only in reuse
+  redraw and one depth read and loses one failed reuse attempt, which is
-  attempts. Across `a92c6ac` drawn widgets, widget draws and primitive
+  the root going through the walk; drawn widgets, widget draws, draw
-  writes are identical on every phase, and only the scheduling counters
+  requests and primitive writes do not move.
-  move.
 
-**None of that reaches seeds 1121 and 1839 at depth 4**, which fail on this
+Everything above is verification of what was changed, not a claim that the
-head and on `ea6dbae` alike. Everything below is verification of what was
+branch is correct.
-changed, not a claim that the branch is correct.
 
 **A claim about a render holds for the commit it was checked at and no
 further.** `tabs` changed twice across `d3b0ebf` with nobody looking; take
@@ -993,12 +966,10 @@ The replay used for the reference check:
 
 In order, from the review above and Bryan's steer (2026-09-17):
 
-1. **A resize marks the root and goes through the walk**; delete
+1. ~~A resize marks the root and goes through the walk.~~ Landed as
-   `dirty_size_under` at both call sites; write `active.parent` where
+   `a0693ac` and `e44dea3`; see the two sections above. The re-parenting
-   `draw_inner` writes `given`, and descendants' `depth` in
+   half turned out to be two defects rather than the predicted one, and
-   `remap_subtree`; pin re-parenting with a test that moves a child between
+   neither was the `depth()` assertion.
-   two spans. Small, argued by the induction above, verified by the rig's
-   counters.
 2. **Frame and extent**, as written above. This is the fundamental change
    and comes before anything built on the placing ask. It lands the two
    parked branches' tests (the stack test unchanged, `Inset`/`Outset` with