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/LAYOUT_LOG.md

@@ -83,7 +83,7 @@ the tail is placed at `(215, 0)..(395, 100)`. Its slot was 210..400, a gap
 too long and a gap too early, and the declared 180 was then centred in it.
 The row reports a total that ends at 400. A child drawn with `rel` lengths
 is stretched into the extra gap instead of being centred in it, because the
-slot is a `Place::Fill`.
+slot fills.
 
 Recomputing the cursor in that branch fixes it, and the tail lands at
 `(220, 0)..(400, 100)`:
@@ -136,9 +136,10 @@ The descriptions above preserve the original failure at `cadfba0`.
 
 ### Clarity, in the order worth doing
 
-The first three -- the unswept `extent`, the two same-typed boxes on
-`ActiveData`, and `Painter`'s four holds accumulators -- were done in
-`5642f20`, and `Part::All` in `aeb60e5`. What is left:
+Everything about naming is done: the unswept `extent`, the two same-typed
+boxes on `ActiveData` and `Painter`'s four holds accumulators in `5642f20`,
+`Part::All` in `aeb60e5`, and `Place`/`Part` themselves in `58ce74d`. The
+settled vocabulary and the ask API are in `docs/LAYOUT.md`. What is left:
 
 - `draw_inner` returns `(Size, LayoutHolds, LayoutHolds)`, two same-typed
   values ordered by convention. A named pair makes the swap unwriteable.