Record narrow_rel_base and the None argument

docs/HANDOFF.md

@@ -6,10 +6,11 @@ in `docs/LAYOUT_LOG.md`.
 
 ## The Iris layout repair is submitted
 
-**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `aeb60e5`:
+**Iris PR #19** (`layout/one-ask`) replaces closed #18. The tip is `beb1386`:
 `add6774` fixes collapsed-share placement, retained mask ownership and a
 redraw-on-reparent defect, `84dad21` removes repeated work in the test
-harness, and `5642f20` and `aeb60e5` are the vocabulary and API work below.
+harness, and `5642f20`, `aeb60e5` and `beb1386` are the vocabulary and API
+work below.
 The core design remains sound; round-to-nearest is still unchanged.
 
 Two checkouts share one Git storage. `/home/bob/repos/iris` is the active
@@ -50,6 +51,9 @@ had been paid for and ergonomics had not:
 - `Painter::place_at` takes the rel base, returns the answer, and draws the
   child where there is no answer to re-express. `Span`'s second pass is one
   call, and its `drawn_across` bookkeeping is gone.
+- The rel base a container decides for a child is `narrow_rel_base`, and it
+  takes `impl Into<Option<[Option<Len>; 2]>>`, so one that narrows nothing
+  writes `None`. The resolved value stays the bare `rel_base`.
 - `Part::All` is a `Part::WHOLE` constant rather than a variant. It was
   exactly `Of(UiSpan::FULL)`; measured at 0.07% of instructions retired
   against 0.04% run-to-run noise on a 60-seed depth-5 shrink.