Update alignment handoff

docs/HANDOFF.md

@@ -8,11 +8,9 @@ log.
 
 Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18
 `split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`; its local
-head is **`8220a78`**, forty-nine commits, pushed to the fork. Built-in
-alignment is **in the working tree at that checkout and not committed** -- see
-"Built-in alignment, in flight" below before touching it. No PR reviews
-were present when checked on 2026-09-15; the verification summary is posted on
-the PR.
+head is **`d3b0ebf`**, fifty commits. Built-in alignment is complete there;
+see "Built-in alignment" below for the retained-layout details. No PR reviews
+were present when checked on 2026-09-15.
 
 The current head completes LAYOUT.md §2's position chain and the requested
 `leftover` behavior. A child whose length is only `leftover` is not drawn when
@@ -72,13 +70,20 @@ rewrite:
 `core/src/ui/holds.rs`, the retained tests, and the generated cold-layout oracle
 pin those rules. Seeds 10 and 86 are now in the ordinary generated set.
 
-## Verification at `8220a78`
+## Verification at `d3b0ebf`
 
 - `cargo fmt --all --check`
+- `cargo build --workspace --all-features`
 - `cargo clippy --workspace --all-targets --all-features -- -D warnings`
-- `cargo test --workspace --all-features`: 85 passed, 10 ignored
-- `cargo test --release --test generated -- --ignored
-  a_long_run_of_seeds_agrees --exact`: 100 seeds passed in 67.6 s
+- `cargo test --workspace --all-features`: 88 passed, 10 ignored
+- The release generated cold-layout oracle passed 100 seeds in 65.5 s.
+- The release shrinker passed both `repaint` and `resize` at 300 seeds each:
+  26,001 widgets per case, largest tree 331 widgets.
+- `tests/drift.rs` passed its 20,000-move exactness check in release mode.
+- The seeded `random` example, live-resized from 1920x1200 to 1280x800, is
+  byte-identical to a cold 1280x800 render. Both PNGs hash to
+  `1d397c57b9914a2e596fa907029bc6aab4629e4d74deb0715e81325c703bcdb3`.
+  The run used the Venus adapter backed by the host RX 7900 XT.
 - `minimal`, `text` and `view` render byte-identical at 1920x1200 across
   `8220a78`. `tabs` differs only in the widget count it prints about itself,
   which is two wrapper types smaller -- so it is no longer a byte-identical
@@ -90,7 +95,13 @@ pin those rules. Seeds 10 and 86 are now in the ordinary generated set.
   before and after replay. Every comparison had zero differing pixels. The
   live-resize image is also byte-identical to the cold 900x1200 image.
 
-The final pre-submit review was run in four passes. The region-node review
+The built-in-alignment pre-submit review was run in four passes. It caught
+three distinctions the smaller tests had missed: an answer's validity must
+include the drawing made in its final placed box; a region-node box is resolved
+through its parent move rather than through its own move twice; and boxes with
+equal dimensions but different positions still need the parent that placed
+them. The 100-seed oracle and both 300-seed shrink cases pass after those
+fixes. The earlier region-node review
 caught an index-reuse hazard when removing a node; its move entry now remains
 alive until every descendant has migrated. The generated oracle then exposed
 the exact `Span` threshold described above. The earlier retained-layout
@@ -134,7 +145,17 @@ tighter figures.
   not, and `Scroll` never reasserts it while drawing.
 - The first box a parent asks about is the offer. A later box chosen from the
   child's answer is the final box, not another independent answer. Dirty
-  widgets are re-asked at the offer and only then drawn in the final box.
+  widgets are re-asked at the offer and only then drawn in the final box. An
+  offer composes through its ancestors' offers, not through their current
+  placed boxes.
+- An answer is reusable only where both its measurement and the drawing made
+  in its final placed box remain valid. The final drawing's `Holds` interval
+  is translated back into lengths of the offered box and intersected with the
+  answer's interval.
+- Equal box lengths do not imply equal placement. An ordinary widget whose
+  offered and current boxes differ in position must involve its parent again;
+  a region node can settle itself only when its own alignment, rather than a
+  container override, determines the final box.
 - A retained drawing can be reused only when its `Holds` interval contains the
   new pixel box on both axes, its parent node is unchanged, its region-node
   choice matches the retained structure, and the widget is clean. A valid
@@ -176,13 +197,11 @@ tighter figures.
   being paid for. The generated oracle compares within 0.05 and `unsettled.rs`
   runs too few frames to reach it, which is why `tests/drift.rs` exists.
 
-## Built-in alignment, in flight
+## Built-in alignment
 
-Uncommitted in `/home/bob/repos/iris-pr18` on top of `8220a78`: 87 tests, fmt
-and clippy clean, the 100-seed generated oracle green, and the shrinker's
-`resize` case green at 300 seeds. The shrinker's **`repaint` case is red**, for
-the reason in "Placement cannot be applied after the fact" below. Its `reorder`
-case is red at `8220a78` too, so that one is **pre-existing and unrelated**.
+Committed as `d3b0ebf` in `/home/bob/repos/iris-pr18`. The shrinker's
+`reorder` case remains red at its base `8220a78`, so that failure is
+pre-existing and unrelated.
 
 What is in it: `align` is a widget property beside `region_node` and the size
 rule, `Aligned` is deleted, `.align()`/`.center()` set the property, and the
@@ -192,10 +211,10 @@ runtime. Region nodes had **no generated coverage at all** before that.
 ### Alignment is two fractions, not four directions
 
 Decided 2026-09-15 (Bryan). A widget's alignment is one `f32` per axis, so a
-quarter of the way along an axis is expressible. `AxisAlign`'s three cases
-become named constants over that number, which is what every expression
+quarter of the way along an axis is expressible. `AxisAlign`'s three familiar
+positions are named constants over that number, which is what every expression
 already uses: the layout math only ever reads `AxisAlign::rel()`, so nothing
-downstream changes shape. The in-flight tree still has the enum.
+downstream changes shape.
 
 The default is **the middle on both axes**, because the two edges are the ones
 that assume a direction -- which edge is the near one depends on the writing
@@ -220,7 +239,7 @@ state carries it.
   the top widget's `placed` is recomputed while an ancestor's shift carries the
   whole subtree. Measured 7,048,813 where a cold layout says 456.
 
-The replacement, not yet written: alignment applies in the two places that
+The replacement applies alignment in the two places that
 already exist and are exact. Where the size is known before drawing, from a
 rule, `declared_box` hands the child its aligned box directly -- one draw, no
 move. Where the size is only known after drawing, the widget is **re-asked in