Record why a move recomposes, and settle alignment's ownership

The measurement is the part worth keeping: adding an offset to a moved
subtree's stored regions drifts linearly and without bound, so the fraction
path is there for accuracy, not generality. `tests/drift.rs` on the Iris
branch pins it.

Built-in alignment and size moves onto #18 rather than waiting for it, with
ownership agreed -- properties on the `region_node` mechanism, a declared
length winning per axis, and a size rule that carries min/max/clamp so a
clamp is resolved where the box is decided instead of by a widget reading
its own pixels.

docs/HANDOFF.md

@@ -8,7 +8,7 @@ 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 **`71c9c39`**, forty-seven commits, pushed to the fork. No PR reviews
+head is **`0283c9d`**, forty-eight commits, pushed to the fork. No PR reviews
 were present when checked on 2026-09-15; the verification summary is posted on
 the PR.
 
@@ -70,11 +70,11 @@ 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 `71c9c39`
+## Verification at `0283c9d`
 
 - `cargo fmt --all --check`
 - `cargo clippy --workspace --all-targets --all-features -- -D warnings`
-- `cargo test --workspace --all-features`: 84 passed, 10 ignored
+- `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 64.77 s
 - The `tabs` reference at 900x1200 remained byte-identical to `f437495`: zero
@@ -158,6 +158,19 @@ tighter figures.
 - `Scroll` reports its content's first measured size. Its drawing can survive
   container-length changes only over the interval in which clamping and its
   current offset do not change.
+- **A move must re-express each part as the same fraction of its new box, not
+  add an offset to the last answer.** A subtree's stored regions are the only
+  record of where it is, so an offset integrates its own rounding and nothing
+  recomputes it. Measured 2026-09-15 on `tests/drift.rs`: offsetting both ends
+  of a span shortens that fixture's row by 0.071 over 20,000 moves and 0.712
+  over 200,000, growing with the count; placing the far end from the near one
+  leaves 0.069, because the length is re-derived from the endpoints either
+  way. 20,000 moves is five minutes of scrolling at 60Hz, which is when it
+  passes the 0.05 physical pixels layout treats as the same place. The
+  fraction path is exact at 200,000. So `RegionRemap` is load-bearing for
+  accuracy rather than for generality, and its multiplies are not what is
+  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.
 
 ## Rigs and reproduction
 
@@ -213,15 +226,48 @@ instruction totals.
 
 ## Next
 
-Let PR #18 review. The next small LAYOUT.md §2 item is `LazySpan`. Region nodes
-now cover the independently movable-subtree use case; do not restore a
-separate child-placement API.
+The next small LAYOUT.md §2 item is `LazySpan`. Region nodes now cover the
+independently movable-subtree use case; do not restore a separate
+child-placement API. `docs/LAYOUT.md` §2 is stale: it still describes
+`Painter::place`, which `71c9c39` replaced.
 
-After #18 lands, the next structural design is built-in alignment and size.
-Today a declared size and the `Size` returned by `draw` are two sources of
-truth, and alignment can disappear behind a wrapper. Agree their ownership
-before implementing it. Do not restore `OnResize::Translate`; retained
-translation is now expressed by the same `Holds` contract and box chain.
+**Built-in alignment and size is next, and goes on #18 rather than after it**
+(Bryan, 2026-09-15: #18 is unreviewed and already large enough that most lines
+get read anyway). Ownership is agreed:
+
+- Declared size and alignment become **widget properties on the same mechanism
+  as `region_node`**, read by `Painter::widget_at`. `SetSize`, `MaxSize` and
+  `Aligned` are deleted, and `.width()`, `.sized()`, `.max_width()`, `.align()`
+  and `.center()` stop building widgets. `Painter` then owns measure-then-place,
+  which `Span`, `Aligned` and `Scroll` each implement separately today.
+- A **declared length wins per axis**; the `Size` returned by `draw` answers
+  only the axes with no declaration. The rule becomes an enum per axis --
+  `Exact`, `Min`, `Max`, `Clamp` -- so a clamp is resolved where the box is
+  decided instead of by a wrapper. `MaxSize::draw` currently calls
+  `px_size()`, which pins its interval to one exact box on both axes and
+  redraws its whole subtree on any resize; the parent resolving the clamp
+  reads px itself and can declare the interval over which the answer holds.
+- The clamp boundary is a **hard layout decision, not a tolerance**, the same
+  shape as `Span`'s leftover split. Its `Holds` range must be exact and split
+  at the crossover; generated seed 16 is what a tolerant endpoint costs.
+- Two things to settle while implementing. `max_width` today does not
+  constrain anything -- `MaxSize` draws its child in its full box and only
+  caps the `Size` it reports upward -- so resolving the rule in the box is a
+  behaviour change. And `declared_len` derives from `size_hint`, so one
+  channel carries both "my size, for a span to divide around" and "resolve my
+  box to this length"; those separate.
+
+Why it comes before anything else that touches layout: a widget's `region`
+becomes **its reported size placed by its alignment inside the offer**, rather
+than the offer itself. A span's measure-then-place then moves a child between
+boxes of the same length, so it is a translation by construction, and the
+measuring draw disappears wherever the size is knowable without one. That
+changes what `.background()` sits behind, so the reference renders stop being
+byte-identical to `ca2b4b2` and the generated cold-layout oracle becomes the
+verification.
+
+Do not restore `OnResize::Translate`; retained translation is now expressed by
+the same `Holds` contract and box chain.
 
 Other queued work, in dependency order: