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.md

@@ -387,7 +387,7 @@ can name -- a division of two lengths, which a `rel + px` length cannot hold.
 A widget's own declaration *is* a fraction, of whichever of those reached it,
 and it is the only one of them that also places the box inside the part.
 
-Validity has a pin for each: `LayoutHolds::extent_len` for the box's symbolic
+Validity has a pin for each: `LayoutHolds::region_len` for the box's symbolic
 length and `frame_len` for the frame's. A range of window pixels cannot say
 which frame an answer is a fraction of, since two frames are different
 lengths at the same window size. Resolving any of these lengths in pixels is