Record what the review of Iris #19 found

Two live defects at cadfba0, each with the repro that showed it and the
fix that answered it: a span places the slot after a collapsed leftover
child a gap early and a gap too long, and a child reused under a parent
that redrew keeps the mask index that parent replaced, so its drawing is
clipped where the subtree used to be. Neither is covered by a test, which
is why the suite is green over both.

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

docs/HANDOFF.md

@@ -1,8 +1,8 @@
 # Handoff
 
-Where the work in flight stands. The durable layout design, the measurement
-method and the findings that outlived the working log are in
-`docs/LAYOUT.md`.
+Where the work in flight stands. The durable layout design and the
+measurement method are in `docs/LAYOUT.md`; what the review of #19 found is
+in `docs/LAYOUT_LOG.md`.
 
 ## The Iris layout repair is submitted
 
@@ -39,23 +39,25 @@ means it was not the software fallback.
 
 ## What is next, in order
 
-1. **Bryan's review of #19.** Fixes to it are unreviewed code: repeat the
+1. **The two defects the review of #19 found**, written up with their
+   repros and fixes in `docs/LAYOUT_LOG.md`: a span misplaces the slot after
+   a collapsed `leftover` child, and a reused child keeps the mask its
+   parent replaced. Both are live at `cadfba0` and neither has a test.
+2. **Bryan's review of #19.** Fixes to it are unreviewed code: repeat the
    `pre-submit-review` passes over whatever each round changes, and re-run
    the three fuzzers and the dump diff for anything that touches `Span`,
    `Painter` or `render_state`.
-2. **The app's Iris pin**, once #19 lands. It is still on the old submodule
+3. **The app's Iris pin**, once #19 lands. It is still on the old submodule
    commit, and the branch changes API the app uses: `SizeRule` beside a
    widget rather than a wrapper, alignment as a widget property, `rest`
    renamed to `leftover`, `OrthoSize` gone.
-3. **Round-to-nearest**, CPU and shader together as one verified change.
+4. **Round-to-nearest**, CPU and shader together as one verified change.
    Bryan approved it on 2026-09-17 and neither half has landed; the
    derivation, the form to use and what to re-check are in `docs/LAYOUT.md`
    under "Rendering the grid (pending)".
 
 ## Smaller layout items, none urgent
 
-- An undrawn `leftover` child still contributes its gap, so a vanished child
-  leaves a double gap.
 - Nested spans pass `leftover` weight up, so three leftover children in one
   inner span beside one in another get three quarters to one quarter. No
   other layout system does that; confirm it is wanted.