Record the rule/widget split for bounds

A rule holds what a widget answers and a widget holds the box (Bryan,
2026-09-20). `layout/bounds` is green on all three scans; the hole left is a
retained answer re-placed under a different rel base, which predates bounds.

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

docs/HANDOFF.md

@@ -200,15 +200,15 @@ glyphs do not follow a shortened entry.
   `Painter::region()`, `SetSize`, `desired_width`, `apply_rest`, `Len::dp`,
   `Aligned` and `MaxSize`, none of which exist. Do not restore
   `OnResize::Translate` or `OrthoSize`.
-- `SizeRule::{Min, Max, Clamp}` is written and pushed as `layout/bounds`
-  (`76aaf06`), and **is not ready to land**: every hand-written test passes,
-  including a capped scroll taking its viewport from the cap, but the
-  400-seed scan does not, and the reason is a design question Bryan has to
-  settle -- see `docs/LAYOUT_LOG.md` under "Bounds are a rule that reads its
-  box". Bryan settled two things already: `Max` narrows the child's drawing
-  box (2026-09-20), and a rule gives a length or bounds one but never both,
-  since a capped share is said with two widgets. A cap may not contain
-  `leftover`.
+- **`layout/bounds` (`de1eb7e`) is green and ready to review**, kept off #19
+  so that branch stays one subject. A rule holds what a widget answers
+  (`SizeRule::{Min, Max, Clamp}`); a widget holds the box (`MaxSize`, which
+  `.max_width`/`.max_height` build). Bryan settled the split on 2026-09-20
+  after four readings were measured; the reasoning and the one remaining hole
+  -- a retained answer re-placed under a different rel base, which is older
+  than bounds -- are in `docs/LAYOUT_LOG.md`. A bound may not contain
+  `leftover`, and the generated trees grow bounds in pixels until that hole is
+  closed.
 - `LazySpan`.
 - `Scroll` taking a direction rather than one axis.
 

docs/LAYOUT_LOG.md

@@ -70,6 +70,25 @@ to it: a rule that is a fraction must pin its rel base, which `Exact` was
 already missing for bounds-shaped reasons; and `widget_trait!` dropped
 attributes, so no method it defines could carry a doc comment.
 
+**Settled (Bryan, 2026-09-20): a rule holds the answer, a widget holds the
+box.** `de1eb7e`. `SizeRule::{Min, Max, Clamp}` holds the length a widget
+answers and leaves its box alone, which is stable because an answer is a
+function of what the widget drew. `MaxSize` is the box version and is a widget
+because a widget is drawn again whenever its own box changes, so the
+comparison is made where the answer can be kept. `.max_width`/`.max_height`
+are that widget; `.min_width`/`.min_height` stay a rule. All three seed scans
+pass. The capped scroll -- 400 px of content under a 100 px cap, a 100 px
+viewport and 300 to scroll -- is the app's `MaxSize` capability, and it comes
+back through the widget.
+
+One hole is left and is **older than bounds**: `place_at` hands a parent a
+retained answer without checking that the answer still holds for the rel base
+that place gives, so a fraction resolved against one rel base survives into
+another. A fraction in a bound is the first thing to reach it (seeds 4 and 196
+at depth 5), which is why the generated trees grow bounds in pixels only, said
+where the tree is grown. An `Exact` rule that is a fraction can reach it too.
+The fix is a holds check at the re-place site, not anything about bounds.
+
 ## Images in the generated trees (2026-09-20)
 
 `Image` is the only widget here whose size hint is a length in pixels, so it is