Move the settled layout vocabulary into LAYOUT.md and prune the handoff

The handoff had grown a chronicle of what each commit renamed. What is
durable about it -- the three names, and the one argument a container uses
to say them -- is design, so it goes to `docs/LAYOUT.md` as its own section
and the handoff keeps a pointer plus where the branch stands.

`docs/PLAN.md`'s "frame/extent experiment" entry described a prototype in
retired words; it becomes a pointer, and the one finding it held that was
not recorded elsewhere -- why the extent-relative child-frame trial was
removed -- joins the failed hypotheses in `docs/LAYOUT.md`.

docs/HANDOFF.md

@@ -1,92 +1,68 @@
 # Handoff
 
-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`.
+Where the work in flight stands. The settled layout design, the vocabulary
+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
 
-**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `58ce74d`:
-`add6774` fixes collapsed-share placement, retained mask ownership and a
-redraw-on-reparent defect, `84dad21` removes repeated work in the test
-harness, and `5642f20` through `58ce74d` are the vocabulary and API work
-below.
-The core design remains sound; round-to-nearest is still unchanged.
+**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `58ce74d`,
+and past the reviewed `cadfba0` it is two rounds:
 
-Two checkouts share one Git storage. `/home/bob/repos/iris` is the active
-`layout/one-ask` worktree; `ai-app-2/iris` stays on `main` at the app's
-`32f6ad8` pin, and is kept until the app's integration is ready.
+- **The repair.** `add6774` fixes collapsed-share placement, retained mask
+  ownership and a redraw-on-reparent defect; `84dad21` removes repeated work
+  in the test harness.
+- **The vocabulary and the container API**, `5642f20` through `58ce74d`. What
+  they settled is described as design in `docs/LAYOUT.md` under "Three names,
+  and the one argument that says them" -- region, placement and rel_base, and
+  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.
 
-Each round was checked with format, workspace clippy under `-D warnings`, the
-workspace tests with and without `layout-diagnostics`, the release fast
-oracle, and the three prescribed seed scans — 400 at depth 5, 1,000 at depth
-6, 2,000 at depth 4. The cold dump covers 34,488 boxes: the fixes moved 650 of
-them, all from the collapsed-share correction, and nothing since has moved
-one.
+The core design remains sound. Round-to-nearest is still unchanged.
 
-## The vocabulary and the container API are settled
+Two checkouts share one Git storage: `/home/bob/repos/iris` is the active
+`layout/one-ask` worktree, and `ai-app-2/iris` stays on `main` at the app's
+`32f6ad8` pin until the integration below is ready.
 
-`5642f20` finished the rename decided on 2026-09-17, which `frame`'s return
-as a length had left half-applied, and `aeb60e5` finished the word `frame`
-itself. Three names, and only two of them are boxes:
+### How each round was checked
 
-- **region** -- the box a parent asks a widget in. It draws there, and a text
-  wraps at it. Was `Painter::extent`, `ActiveData::part`, `DrawInfo::part`,
-  `Placing::extent`, `LayoutHolds::extent`.
-- **placement** -- the box its drawing ends up in, the size it reported,
-  once it has answered. Equal to the region wherever it fills one. Was
-  `ActiveData::extent`; `placed_extent` is now `placement`.
-- **rel_base** -- not a box: the length a fraction it declares or reports is
-  a fraction of. Was `frame`, which said the wrong shape.
-  `Painter::frame_len()` is `Painter::rel_base()`, `LayoutHolds::frame_len`
-  is `rel_base`, and `frame_and_extent` is `rel_base_and_region`.
+Format, workspace clippy under `-D warnings` with and without
+`layout-diagnostics`, the workspace tests both ways, and the three prescribed
+seed scans -- 400 at depth 5, 1,000 at depth 6, 2,000 at depth 4.
 
-`aeb60e5` also cut what a container has to say, since correctness and cost
-had been paid for and ergonomics had not:
-
-- **One argument says where a child goes.** `widget_at` and `place_at` take
-  `impl Into<PlaceDesc>`, so a wrapper passes a `UiRegion` and says nothing
-  else -- `Pad` and `Offset` are one call each. `Place` and `Part` are gone:
-  `Place` was a product written as a sum, and `Part`'s three cases named
-  operations the geometry already had (`UiSpan::within`, `UiSpan::shift`,
-  and `placement`'s own body).
-- **`PlaceDescAxis` is named after the operation**: `within`, `shifted`,
-  `sized`, `WHOLE`, with `fills` and `rel_base` as builders. The rel base a
-  caller does not state follows the constructor -- `within` narrows it,
-  `shifted` passes it through, `sized` is it -- which removes the one rule a
-  caller could get wrong with nothing failing.
-- **`PlaceDesc` is a proper pair**, with `x`/`y`, `axis`, `axis_mut` and
-  `from_axis` like every other pair here, so joint work is written once
-  rather than per axis.
-- `Painter::place_at` returns the answer and draws the child where there is
-  no answer to re-express, so `Span`'s second pass is one call and its
-  `drawn_across` bookkeeping is gone.
-- `widget_within` and `ActiveData::narrow_rel_base` are deleted; `asked` and
-  `placed` carry the rel base their ask stated.
-
-Cold layout is byte-identical to `84dad21` across all of it, over 400
-depth-5 trees.
+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.
 
 ## What is next, in order
 
 1. **Bryan's review of #19.** Fixes are themselves unreviewed code: repeat
    `pre-submit-review` over each round's changes, and re-run the three
-   fuzzers and dump diff for edits to `Span`, `Painter`, or `render_state`.
-   The ordinary oracle does not replace absolute geometry and
+   fuzzers and the dump diff for edits to `Span`, `Painter`, or
+   `render_state`. The ordinary oracle does not replace absolute geometry and
    retained-primitive expectations.
 2. **Integrate the app's Iris capabilities before changing its pin.**
    `32f6ad8` has 45 commits not reachable from the review branch; shared UI
    ownership, richer masks, Android support, and app-side performance work
    must survive the integration. The app then absorbs the branch's
-   vocabulary and container API, all of it already upstream: `rest` →
+   vocabulary and container API, all of it already upstream: `rest` ->
    `leftover`, `SizeRule` beside widgets, alignment properties, the removal
-   of `OrthoSize`, the region/placement/rel_base names above, and
-   `widget_within`/`place_at`.
+   of `OrthoSize`, region/placement/rel_base, and `PlaceDesc`.
 3. **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)".
 
+Wanted but not started, recorded in `iris/TODO`: transforms on a move entry,
+so a whole subtree scales or rotates with one buffer write and no redraw.
+Compose-style stretch at the end of a scroll area is the use that prompted
+it. A move entry only translates today, and composing through one scales the
+`rel` part while `px` passes through untouched, so fixed-size content and
+glyphs do not follow a shortened entry.
+
 ## Smaller layout items, none urgent
 
 - Nested spans pass `leftover` weight up, so three leftover children in one