Note that transparent leftover across a span is deferred

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -178,7 +178,12 @@ when no child has a `px` or `rel` part does the span report `leftover`
 itself and take its parent's room (Bryan, 2026-09-18: "returning rest if
 any have it is probably fine", refined to this because it costs nothing).
 So `row![text, rect]` is as tall as the text with the rect filling it, and
-`row![rect, rect]` fills its column's share.
+`row![rect, rect]` fills its column's share. **Deliberately not done**: a
+fully transparent `leftover` across a span, where the span's height would
+be the larger of its tallest fixed child and the share its parent hands
+back. That needs the parent's division to come back down after the report,
+a second resolution pass; Bryan deferred it until the core is settled. Do
+not attempt it as part of this plan.
 
 The shapes the rest of the containers take (write them, they are short):