Record the PlaceDesc argument in the handoff

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 `c55be21`:
+**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 `c55be21` are the vocabulary and API
-work below.
+harness, and `5642f20` through `58ce74d` 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
@@ -44,19 +44,25 @@ itself. Three names, and only two of them are boxes:
 `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.
-- The rel base a container decides for a child is `narrow_rel_base`, and it
-  takes `impl Into<Option<[Option<Len>; 2]>>`, so one that narrows nothing
-  writes `None`. The resolved value stays the bare `rel_base`.
-- `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.
+- **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.