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/LAYOUT_LOG.md

@@ -138,7 +138,7 @@ The descriptions above preserve the original failure at `cadfba0`.
 
 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:
+`5642f20`, and `Part::All` in `aeb60e5`. What is left:
 
 - `draw_inner` returns `(Size, LayoutHolds, LayoutHolds)`, two same-typed
   values ordered by convention. A named pair makes the swap unwriteable.
@@ -152,9 +152,3 @@ The first three -- the unswept `extent`, the two same-typed boxes on
   `under.iter_mut().find`, `depend_on`), so a span of *n* children is
   O(n^2) per draw. Not a problem at today's sizes; it is worth knowing
   before a long transcript list lands on it.
-- `Part::All` is exactly `Part::Of(UiSpan::FULL)`: `Of` composes through
-  `within`, and `through(Len::FULL)` maps a range back to itself, which
-  `the_whole_of_a_box_maps_back_to_itself` already asserts. It buys a
-  separate arm in `in_parent` and in `Part::of`. Keeping it as sugar is
-  defensible; the case analysis being wider than the geometry is the thing
-  to decide about rather than to leave unstated.