Document independent widget bounds and measured layout costs

docs/LAYOUT.md

@@ -12,6 +12,57 @@ in flight stands. The sections from "Three names, and the one argument that
 says them" onwards are the settled design, the findings that outlived the
 working log, and the measurement method.
 
+## Bounds on the offered box
+
+`SizeRule` carries an optional preferred request and independent min/max bounds.
+The bounds constrain both the box offered to a widget and its reported size.
+`.min_width`, `.max_width` and their height variants edit that widget's rules
+and preserve its identity and type; `MaxSize` is removed. Setting `.width` or `.height` preserves the bounds; setting a bound
+preserves the preferred request and the other bound, in either call order.
+`Widgets::set_len` makes the same edit at runtime; `set_size_rule` replaces
+the complete axis. This replaces the rule/widget split recorded in
+`LAYOUT_LOG.md` (Bryan, 2026-09-20).
+
+The shared ask resolves declarations and bounds against the incoming rel
+base, compares the offer, and retains the chosen box length in `Declared`.
+These are window lengths. Placement aligns a retained length in its destination
+without resolving it again or comparing the destination against the bound.
+The comparison's window range and incoming-region dependency invalidate the
+ask when its inputs change. Ask constraints are combined after the draw, so a
+widget widening its own `window_holds` cannot erase a bound's crossing.
+Fractional rules also track their incoming base. Allocated slots use the
+allocator's base for bounds; applying a 25% cap to its own 25% slot would
+shrink it twice. Changing an inactive bound also reaches the parent, because
+it changes which offers the parent can safely reuse.
+A cap constrains wrapping and scroll viewports, but short intrinsic content
+can still report less than the cap. Shares remain deferred requests for the
+parent to allocate; the offered box does not replace their reported weight.
+
+Measured against `4cb6f68`, medians of five release-process instruction counts:
+
+| Fixture | Before | After | Change |
+| --- | ---: | ---: | ---: |
+| 128 capped rows, 2,000 resizes | 7.387B | 5.779B | -21.77% |
+| Same, every widget redrawn | 13.864B | 9.755B | -29.64% |
+| 128 plain rows, every widget redrawn | 10.041B | 10.010B | -0.31% |
+| 128 fixed-width rows, every widget redrawn | 10.085B | 10.056B | -0.29% |
+| 40 text rows, 1,000 resizes | 9.460B | 9.472B | +0.13% |
+
+Plain and fixed-width retained resizes differ by under 0.05%; text repaint,
+edit and scroll differ by -0.05%, +0.07% and +0.06%. `tests/bounds_cost.rs`
+asserts the same geometry for the old builders and new attributes, with 513
+active widgets before and 385 after in the capped fixture. The text runs use
+`tests/revision_cost.rs`. These include cold setup and do not measure GPU or
+phone frame times. The unbounded cold dump remains identical at 34,986 boxes;
+bounded geometry changes intentionally because the offer is now constrained.
+
+The storage tradeoff is explicit: a `SizeRule` is now 40 bytes rather than 24,
+so independent preferences and bounds add 32 inline bytes per widget across
+both axes. Plain, expression-bounded and attribute-bounded fixtures each
+perform zero allocations over 100 forced-redraw resize frames after warm-up.
+Bounds introduce no heap allocation; an expression also loses
+its former extra outer `Arc`. Artifacts are in `/tmp/attribute-bounds/`.
+
 ## Deferred size requests on PR #19
 
 `Widget::draw` still returns the small, copyable `Size` of two `LayoutLen`s.