Record that a stack's sizing child decides its siblings' boxes

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

docs/HANDOFF.md

@@ -191,8 +191,9 @@ before landing and inspect any change.
 
 ### Frames are narrowed by every length decided from above
 
-A declared `px` or `rel`, a resolved share, and (new, planner's choice, not
-yet Bryan's) the box a stack's sizing child decided all narrow the frame.
+A declared `px` or `rel`, a resolved share, and the box a stack's sizing
+child decided (Bryan, 2026-09-18: the sizing child, if any, determines how
+the rest are laid out) all narrow the frame.
 `declared_lens` still excludes `leftover`, which is right: a share has no
 length until the span divides its room, and it narrows the frame at the
 placing ask instead.