Say what a leftover means where nothing divides it

Bryan's rule, generalising the max he gave for Scroll's content length on
2026-09-18: a leftover under a parent that does not divide is still a leftover
and acts as a minimum, so the length is max(box, px + rel*box) -- it fills the
rest where the fixed parts are shorter and overflows where they are longer.

Measured against a span, which implements it, and against the non-dividing path,
which drops the overflow in the one row where the fixed part is longer than the
box. Recorded with the cause and the window contract a fix needs; not fixed,
because what a length means is Bryan's to settle and nothing here mixes the two
outside the span's own test.

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

docs/HANDOFF.md

@@ -177,6 +177,12 @@ glyphs do not follow a shortened entry.
   other layout system does that; confirm it is wanted.
 - A span can overflow itself without bound, so boxes of negative length reach
   children and nothing states what a widget may assume about one.
+- A `leftover` under a parent that does not divide is a minimum size --
+  `max(box, px + rel*box)` (Bryan, 2026-09-20). A span does that; the
+  non-dividing path drops the overflow where the fixed part is longer than
+  the box. Measured in `docs/LAYOUT_LOG.md` under the ninth sweep, with the
+  window contract a fix needs. Nothing here mixes the two, so nothing is
+  wrong on screen.
 - `Fixed::div` by zero answers `MIN`/`MAX` while `ratio` answers `ZERO`; both
   are caller bugs under `debug_assert`, but the fallbacks differ.
 - `docs/LAYOUT.md` §4, §5 and the density section name `Painter::place`,