Record what the last of the fixed-point imprecision is

Traced rather than guessed: the same box reached two ways, each rounding
where the other does not. Two of the three are fixed, the third is priced --
it costs the retained resize path, at a thousandth of a pixel.

`OrthoSize` is deleted rather than paused; the decision it was waiting on was
that a span asks whether a rule already answers for it.

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

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 **`cb955f1`**, fifty-five commits. Built-in alignment is complete there;
+head is **`bdab558`**, fifty-seven 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.
 
@@ -72,13 +72,14 @@ 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 `cb955f1`
+## Verification at `bdab558`
 
 - `cargo fmt --all --check`
 - `cargo build --workspace --all-features`
 - `cargo clippy --workspace --all-targets --all-features -- -D warnings`
-- `cargo test --workspace --all-features`: 102 passed, 10 ignored
-- The release generated cold-layout oracle passed 100 seeds in 75 s.
+- `cargo test --workspace --all-features`: 103 passed, 10 ignored
+- The release generated cold-layout oracle passed 100 seeds in 9.4 s, and a
+  shrinker case at 300 seeds in 3.5 s: both run a thread per core but one.
 - The release shrinker passed **all five** cases -- `resize`, `repaint`,
   `resize-repaint`, `reorder`, `size-change` -- at 300 seeds of depth 5:
   26,001 widgets per case, largest tree 331. It passed 1000 seeds of depth 6
@@ -137,6 +138,13 @@ tighter figures.
 
 ## Retained-layout invariants
 
+- A span is as long across itself as its longest child, unless a rule beside
+  it says how long it is -- and then it does not read its children there at
+  all, since the answer is not wanted and reading one is what makes its size
+  depend on it. `OrthoSize` was that second case written twice and is gone
+  (`9d8415d`); `Painter::ruled` is how a container asks which it is in, and
+  the only thing a widget may learn about a rule over it.
+
 - A region node holds a whole `UiRegion` in its parent node's coordinates.
   `UiRegion::FULL` is the identity. Widgets opt in with `.region_node()` or
   `Widgets::set_region_node`; ordinary widgets share the nearest ancestor
@@ -315,12 +323,24 @@ pixels and `Holds`. Decided with Bryan on 2026-09-15.
   rounding, so it scales with the step: at `1/64` that was 0.016 px, enough
   to move a box, and at `1/1024` it is a thousandth of a pixel. Range is
   +/-2.1M px and conversion to `f32` is exact to 16,384 px.
-- **What the fuzzers ask for is a step per operation.** The shrinker's five
-  cases agree within one (`resize` exactly), the oracle's two-operation cases
-  within two. Closing that needs one way of asking how long a box is: today a
-  chain composed down through `within` and a length measured against the
-  window are two, and they round separately. That is the next real step in
-  precision and it is bigger than any of these four commits.
+- **What the fuzzers ask for is a step per level of nesting**, which is two
+  for these trees. Traced on 2026-09-16 to the same box reached two ways,
+  each rounding where the other does not -- not accumulation, and not one
+  place. Two of them are fixed in `bdab558`:
+  - `Scroll` wrote a box it had been given back out as its own length in
+    pixels. Centring a part in `rel 1` lands a step from centring it in
+    `px 900`, because `a(x - y)` and `ax - ay` do not round alike. Content
+    that fills the viewport unscrolled is handed back as it came, and the
+    `repaint` and `resize-repaint` cases became exact.
+  - `Span` placed each child a step from where the last ended, carrying every
+    share's rounding along the row. A position is now the fixed parts before
+    it, exact, plus one rounded share. Two hundred equal shares of a 1000 px
+    row ended at 999.999 and now end at 1000.
+  What is left is a box centred in a fraction of its parent against the same
+  box centred in its own pixels, one step per level between them. Closing it
+  means alignment resolved in pixels everywhere -- which costs the retained
+  resize path, since it is the fractional form that re-centres a subtree
+  without redrawing it. Not worth it at a thousandth of a pixel.
 - `Holds::through` inverts `px + rel * box`, which rounds, so the answer is
   an interval even for a single length. It maps the half step either side,
   plus one more for the two ways above; inverting the length alone gives a
@@ -376,6 +396,11 @@ targets went from 14.3 s to 7.7 s and `target/` from 45 GB to 13 GB. Pick a
 module out with `cargo test --test suite layout::`. The fuzzers and the
 `*_cost` measurements are still their own targets.
 
+The fuzzers take a thread per core but one (`9d8415d`), since a seed grows,
+lays out and drops its tree alone: the oracle's hundred seeds went from 68 s
+to 9.4 s and a shrinker case at 300 seeds from 18 s to 3.5 s. A failing seed
+still shrinks and panics on its own thread.
+
 Run the long generated oracle only after ordinary tests pass:
 
 ```sh
@@ -433,15 +458,6 @@ the same `Holds` contract and box chain.
 
 Queued from this work, in order:
 
-- Delete `OrthoSize`. It is redundant now that size is built in: a span should
-  read its own rule on the orthogonal axis and, where that is fixed, skip
-  reading its children's orthogonal sizes entirely, which is what `Full` does
-  today. That also removes the span's own instance of the compounding above.
-  **Paused 2026-09-16 for a decision**: a widget deliberately cannot see its
-  own rule ("the widget under a rule never learns of it"), so either that
-  changes or the span reports something else on the orthogonal axis -- which
-  is what every span reports, and a column sized to its widest item is what
-  `Children` is for.
 - `Scroll` should take a direction rather than one axis: vertical, horizontal,
   or both. Reporting `LEFTOVER` on both axes is already the right shape for it.
 - Restore `max_width`/`max_height` as `SizeRule::{Min, Max, Clamp}`. `8220a78`