Record the naming round in the handoff and LAYOUT.md

Iris PR #19's tip is 2807a92. LAYOUT.md's settled-design section named the
retired `PlaceDescAxis::within`/`shifted`/`sized` constructors and the
`axis`/`axis_mut` accessors; it now says the `_desc` chaining rule, the
`axis` lift, and that every pair is a struct of two per-axis values read
with `[axis]`.

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

docs/HANDOFF.md

@@ -6,8 +6,8 @@ found is in `docs/LAYOUT_LOG.md`.
 
 ## The Iris layout repair is submitted
 
-**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `58ce74d`,
+**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `2807a92`,
-and past the reviewed `cadfba0` it is two rounds:
+and past the reviewed `cadfba0` it is three rounds:
 
 - **The repair.** `add6774` fixes collapsed-share placement, retained mask
   ownership and a redraw-on-reparent defect; `84dad21` removes repeated work
@@ -18,6 +18,15 @@ and past the reviewed `cadfba0` it is two rounds:
   the single `impl Into<PlaceDesc>` argument that replaced `Place`, `Part`,
   `widget_within` and the separate rel-base argument. **Bryan is still
   refining this**, so treat the API as current rather than frozen.
+- **Named operations and `Index<Axis>`**, `55df32a` and `2807a92`, settled
+  with Bryan on 2026-09-19. A description is built by chaining off the value
+  that says it (`UiSpan::shifted_desc`, `Len::as_desc`), never by a
+  constructor naming the type; `on` is reserved for events, so the lift is
+  `PlaceDescAxis::axis`. Arithmetic that needed a comment became a name:
+  `UiSpan::place`, `LayoutLen::without_leftover`/`is_px`/`is_only_leftover`/
+  `declared`, `Holds::covers`. `LayoutHolds` became `AxisHolds` on `x`/`y`,
+  every pair got `Index<Axis>` through `impl_axis_index!`, and the bare
+  `[Option<LayoutLen>; 2]` became `Declared` of `Option<Len>`.
 
 The core design remains sound. Round-to-nearest is still unchanged.
 
@@ -35,7 +44,8 @@ The gate that matters for anything shaped like a rename or a refactor is the
 **cold dump**: `layout_dump` over 400 depth-5 trees is 34,492 boxes, and the
 compiler cannot catch two same-typed values being swapped. The repair moved
 650 of those boxes, all from the collapsed-share correction. Every commit
-since has been byte-identical to `84dad21`, including the whole API rewrite.
+since has been byte-identical to `84dad21`, including the whole API rewrite
+and the naming work above.
 
 ## What is next, in order
 

docs/LAYOUT.md

@@ -344,30 +344,41 @@ and `place_at(id, place)` take `impl Into<PlaceDesc>`, so a wrapper that only
 hands over a box passes a `UiRegion` and says nothing else — which is all
 `Pad`, `Offset` and `Painter::widget` do.
 
-`PlaceDescAxis` is one axis, and its three constructors are named after the
+`PlaceDescAxis` is one axis. It is built by chaining off the value that says
-operations the geometry already had, because two of them take the same span
+it, never by a constructor naming the type, because a constructor makes the
-and differ only in how it is applied:
+reader go back to the start of the line. The `_desc` suffix is what says which
+type comes out. The three are named after the operations the geometry already
+had, because two of them take the same span and differ only in how it is
+applied:
 
-- `within(span)` — `UiSpan::within`: composed into the caller's box, so it
+- `UiSpan::within_desc()` — `UiSpan::within`: composed into the caller's box, so it
   moves and scales with it. What an inset speaks.
-- `shifted(span)` — `UiSpan::shift`: window lengths from where the caller's
+- `UiSpan::shifted_desc()` — `UiSpan::shift`: window lengths from where the caller's
   box starts. What a container dividing room speaks, and what makes a moved
   box re-place every child by addition, exactly. Not directional: `Dir::Neg`
   is handled by the span before the numbers get here.
-- `sized(len)` — the body of `placement` with the length given from above
+- `Len::as_desc()` — the body of `placement` with the length given from above
   rather than reported. What a stack's sizing child decides for the rest.
 
 What is optional is a builder, so a caller writes only what it decided:
 `.fills()` says the region is the placement, and `.rel_base(len)` names the
 child's rel base outright. **The rel base a caller does not name follows the
-constructor** — `within` narrows it the way the box is narrowed, `shifted`
+constructor** — `within_desc` narrows it the way the box is narrowed,
-passes it through, `sized` is it. That rule is what makes the common case
+`shifted_desc` passes it through, `as_desc` is it. That rule is what makes the common case
 right by default; stating it by hand was the one thing a container could get
 wrong with nothing failing.
 
-`PlaceDesc` is the pair, with `x` and `y` fields and the `axis`, `axis_mut`
+`PlaceDesc` is the pair, with `x` and `y` fields and the `Index<Axis>` and
-and `from_axis` of every other pair here, so the joint work — resolving a
+`from_axis` of every other pair here, so the joint work — resolving a
 region, reading the fill flags — is written once rather than per axis.
+`PlaceDescAxis::axis(axis)` goes the other way, lifting one axis into a pair
+with the whole box across it.
+
+**Every pair here is a struct of two per-axis values, read with `[axis]`.**
+`impl_axis_index!` gives it `Index<Axis>`/`IndexMut<Axis>`; there are no
+`axis`/`axis_mut` methods and no bare `[T; 2]`. A pair kept as arrays of its
+fields instead — which `LayoutHolds` was, until `AxisHolds` — cannot write
+any of its own operations once.
 
 ## Rel bases, decided boxes and padding