Record the region/placement sweep and prune the handoff

The `extent` rename landed as iris `5642f20`; the handoff had not listed it
as pending at all, and its item 2 named renames that are already upstream.
Say which vocabulary the app has still to absorb, drop the retired-clone
and archive chronicle, and strike the three clarity findings now done.

docs/LAYOUT_LOG.md

@@ -136,18 +136,10 @@ The descriptions above preserve the original failure at `cadfba0`.
 
 ### Clarity, in the order worth doing
 
-- **`extent` is back.** `core/src/ui` says `extent` 138 times, its comments
-  say "box" 153 times, and the types are all `UiRegion`. Bryan renamed this
-  to `placement` on 2026-09-17; `frame` survived with its new meaning as a
-  length, `extent` did not. One sweep over `Painter::extent`,
-  `ActiveData::{extent, part}`, `placed_extent`, `frame_and_extent` and
-  `LayoutHolds::{extent, extent_len}`.
-- `ActiveData::part` and `::extent` are both boxes told apart only by
-  prose, and `draw_at` writes `part: extent` from one value. `asked_in` and
-  `drawn_in` would say it in the names.
-- `Painter` accumulates a `LayoutHolds` under four names that do not match
-  the four fields they become (`window_own`, `extent_own`, `frame_own_len`,
-  `extent_len`). One `own: LayoutHolds` field deletes the mapping.
+The first three -- the unswept `extent`, the two same-typed boxes on
+`ActiveData`, and `Painter`'s four holds accumulators -- were done in
+`5642f20`. What is left:
+
 - `draw_inner` returns `(Size, LayoutHolds, LayoutHolds)`, two same-typed
   values ordered by convention. A named pair makes the swap unwriteable.
 - `DrawInfo::px` is computed at all four construction sites and read only by