Withdraw the Moves::clear item from the thirteenth sweep

Bryan asked what that arena was and whether keeping its capacity had a
point. It does not: Moves::clear runs only when the root changes, beside a
rebuild of the whole tree, where RequestArena::reset runs every frame and
the allocation rig checks it. Arena also holds an IdTracker a clear has to
reset. The parallel was drawn on the shape of the code rather than on how
often it runs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -80,9 +80,10 @@ generator still grows pixels, because `deferred_generated.rs` already varies
 that dimension and growing fractions here would move every box in the cold
 dump; the comment says that instead of describing an open defect.
 
-`docs/LAYOUT_LOG.md` has all seven, with the five things the sweep looked at
-and left. Verified: format, clippy both ways, 208 and 212 tests, and the
-cold dump byte-identical to `f48e04e` across all 34,986 boxes.
+`docs/LAYOUT_LOG.md` has all seven, with the four things the sweep looked
+at and left and the one it withdrew. Verified: format, clippy both ways,
+208 and 212 tests, and the cold dump byte-identical to `f48e04e` across all
+34,986 boxes.
 
 ## A quality sweep of the request arena is submitted on PR #19