Document per-axis retained layout

docs/LAYOUT.md

@@ -106,8 +106,11 @@ stands rather than new machinery:
 and the primitive's stored `rel`/`abs` pair every frame, already, on the GPU.
 A widget laid out purely in `rel`/`abs` terms is therefore already correct
 after a resize with zero CPU work. Calls to `Painter::px_size` and
-`Painter::output_size` mark the active widget as reading concrete pixels;
-only those widgets become dirty when the output actually changes.
+`Painter::output_size` mark both concrete-pixel axes; `px_len(axis)` and
+`output_len(axis)` mark only the axis actually read. Only widgets whose read
+axes changed by more than 0.05 physical pixels become dirty. The comparison
+is against each widget's last actual draw, so smaller changes accumulate
+rather than disappearing event by event.
 
 All pixel-dependent leaves are marked before layout begins, along with every
 chain of parents that read their sizes. Resize then settles the shallowest
@@ -145,18 +148,19 @@ An exact `size_hint` stops propagation when both axes still equal the retained
 size. Otherwise propagation is deliberately conservative: the child may have
 changed size, and only its dependent ancestors can assign the final boxes.
 Unchanged descendants still take `draw_inner`'s retained skip-or-move path.
-For a stacking container, retained child lengths are cached per axis: a child's
-width remains reusable while the parent changes width, and its height remains
-reusable while the parent changes height. A change on the orthogonal axis does
-invalidate it in both directions. This is a generic constraint rule, not a
-text exception; wrapped text is merely the common example of height depending
-on width.
+An active widget also retains which offered-box and output axes flowed into
+the size it reported, directly or through a child size it read. A container
+may use that answer for the same prospective box when every observed input is
+still within 0.05 physical pixels; content dirtiness anywhere in its size
+dependency subtree rejects the answer. This is a generic constraint rule, not
+a text exception. Wrapped text is merely the common example: it reads width,
+so changing only height leaves its answer valid.
 
 ### 4. Wrapped text, and "needs child height before choosing width"
 
 **Wrapped text is not a special case any more; it already reads as one
 draw.** `TextView::render` (`iris/src/widget/text/mod.rs:57-76`) already
-does exactly what single-draw asks for: it reads `ctx.px_size().x` as the
+does exactly what single-draw asks for: it reads `ctx.px_len(Axis::X)` as the
 wrap width, shapes once, and memoizes the shaped layout keyed on that width
 plus a changed-flag on the buffer and attrs (`:63-69`) — a second call with
 the same width is a hash-map-style cache hit, not a re-shape. Under the new