Record rel_base and the container API, and sweep LAYOUT.md

`frame` is gone from the layout vocabulary in `docs/LAYOUT.md` too, since
leaving the design document in the old words defeats the rename; the
rendered-frame sense of the word is untouched. The `local == UiRegion::FULL`
item is dropped from the handoff: that comment was #18's and is not on the
review branch.

docs/HANDOFF.md

@@ -6,11 +6,11 @@ in `docs/LAYOUT_LOG.md`.
 
 ## The Iris layout repair is submitted
 
-**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `5642f20`:
+**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `aeb60e5`:
 `add6774` fixes collapsed-share placement, retained mask ownership and a
 redraw-on-reparent defect, `84dad21` removes repeated work in the test
-harness, and `5642f20` is the naming sweep below. The core design remains
-sound; round-to-nearest is still unchanged.
+harness, and `5642f20` and `aeb60e5` are the vocabulary and API work below.
+The core design remains sound; round-to-nearest is still unchanged.
 
 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
@@ -20,24 +20,42 @@ 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 the naming sweep moved
-none.
+them, all from the collapsed-share correction, and nothing since has moved
+one.
 
-## The naming sweep is done
+## The vocabulary and the container API are settled
 
-`5642f20` finished the rename decided on 2026-09-17, which `frame`'s return as
-a length had left half-applied. The box a parent asks a widget in is the
-**region**, and where its drawing ends up is its **placement**:
+`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:
 
-- `Painter::extent` → `region`, and its `extent_len()` → `region_len()`.
-- `ActiveData::{part, extent}` → `{region, placement}`, and with them
-  `DrawInfo::part` and `Placing::extent`.
-- `LayoutHolds::{extent, extent_len}` → `{region, region_len}`, and
-  `Counter::OutsideExtent` → `OutsideRegion`.
-- `placed_extent` → `placement`, `frame_and_extent` → `frame_and_region`,
-  `part_of` → `region_of`.
-- `Painter`'s `window_own`, `extent_own`, `frame_own_len` and `extent_len`
-  become the one `own: LayoutHolds` they were assembled into.
+- **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`.
+
+`aeb60e5` also cut what a container has to say, since correctness and cost
+had been paid for and ergonomics had not:
+
+- `Painter::widget_within(id, region)` takes a box in the widget's own
+  coordinates and derives the child's rel base from it, so `Pad` and
+  `Offset` are one call each. `Offset` had been reading `region_len`, which
+  pinned its drawing to a box length an offset does not depend on.
+- `Painter::place_at` takes the rel base, returns the answer, and draws the
+  child where there is no answer to re-express. `Span`'s second pass is one
+  call, and its `drawn_across` bookkeeping is gone.
+- `Part::All` is a `Part::WHOLE` constant rather than a variant. It was
+  exactly `Of(UiSpan::FULL)`; measured at 0.07% of instructions retired
+  against 0.04% run-to-run noise on a 60-seed depth-5 shrink.
+
+Cold layout is byte-identical to `84dad21` across all of it, over 400
+depth-5 trees.
 
 ## What is next, in order
 
@@ -50,9 +68,10 @@ a length had left half-applied. The box a parent asks a widget in is the
    `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, all of it already upstream: `rest` → `leftover`, `SizeRule`
-   beside widgets, alignment properties, the removal of `OrthoSize`, and the
-   region/placement names above.
+   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`.
 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`
@@ -67,9 +86,6 @@ a length had left half-applied. The box a parent asks a widget in is the
   children and nothing states what a widget may assume about one.
 - `Fixed::div` by zero answers `MIN`/`MAX` while `ratio` answers `ZERO`; both
   are caller bugs under `debug_assert`, but the fallbacks differ.
-- The comment on the `local == UiRegion::FULL` shortcut in `widget_at` says
-  composing through `FULL` "is not quite the identity in f32"; on the grid it
-  is exact and the shortcut is performance only.
 - `docs/LAYOUT.md` §4, §5 and the density section name `Painter::place`,
   `Painter::region()`, `SetSize`, `desired_width`, `apply_rest`, `Len::dp`,
   `Aligned` and `MaxSize`, none of which exist. Do not restore