Document per-axis retained layout

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -9,7 +9,7 @@ Canonical `main` is **`ca2b4b2`** (#17, the headless rig). Sixteen slices are
 in.
 
 **#18 `split/18-position-chain`** is open and finished apart from one decision:
-worktree `/home/bob/repos/iris-pr18`, head `82fa6c1`, seventeen commits,
+worktree `/home/bob/repos/iris-pr18`, head `b1b3eca`, eighteen commits,
 workspace tests passing, fmt and clippy clean. It is LAYOUT.md §2's position chain, generalised
 to boxes. `/home/bob/repos/ai-app-2` is on `rustify`, worktree clean.
 
@@ -217,9 +217,24 @@ hottest texts draw 32 times. Instrumented layout is about 5.5 ms instead of
 60.79B instructions total (60.8M/frame), and 23.85B cycles. This is about 58%
 fewer instructions than the roughly 138M/frame #18 path before coalescing,
 but still about 1.8x the old 3.14 ms resize median. The remaining multiplier
-is within one constraint traversal, so per-axis retained size validity is now
-the useful next question. Re-sending the existing output size also no longer
-starts a resize.
+is within one constraint traversal. Re-sending the existing output size also
+no longer starts a resize.
+
+`b1b3eca` records pixel reads and retained size inputs per axis. `Text` says it
+reads only its offered width, while `Scroll` says it reads only its scrolling
+axis. `Span`, `Aligned`, and `Scroll` ask for an exact hint or a retained child
+length valid under the box they are about to offer, and draw to measure only
+when neither exists. On the same depth-8 load, resize now averages 449 widget
+draws, 296 placement calls, 485 drawn-size reads, 12,164 primitive writes, and
+107 text shapes. Cached uninstrumented 1,000-frame runs measured 4.95–5.24 ms
+medians, 55.4B instructions total (55.4M/frame), and 21.5–21.6B cycles: about
+9% fewer instructions than `82fa6c1`, but still about 1.6x the old 3.14 ms
+median.
+
+Pixel comparisons now use a 0.05 physical-pixel tolerance. The comparison is
+against the last actual layout, not the preceding resize event or retained
+move, so repeated subpixel changes accumulate and eventually redraw. The
+generated cold-layout oracle uses the same visual tolerance per coordinate.
 
 The generated tree now includes `Aligned` with every meaningful per-axis
 alignment. That exposed two retained-layout ordering bugs which `84f589e`
@@ -227,7 +242,9 @@ fixes. The regular cold-layout equivalence suite and the ignored 100-seed
 sweep pass. The latter previously stopped at seed 60 on `140.0` versus
 `139.99996948242188`: exactly two `f32` ULPs from an equivalent composition
 order, not a visible layout difference. The oracle now keeps draw presence
-exact and allows at most four ULPs per pixel coordinate.
+exact and allows at most 0.05 physical pixels per coordinate. Seed 98 is also
+part of the ordinary suite because it catches stale retained size beneath an
+aligned size-changing chain.
 
 The branches are invariants rather than widget exceptions:
 
@@ -255,6 +272,14 @@ stale-size failure. Re-measuring the scroll subtree costs 11 draws rather than
 1, but remains two orders of magnitude below the old 1,313 and follows the
 actual dependency invariant.
 
+A frame-local cache of several size answers under different constraint boxes
+was also discarded. It reached about 3.5 ms in the instrumented resize rig,
+but seed 98 showed that a valid answer is not enough: the child's one active
+drawing may still realize another constraint, and a parent can then lay out
+from an answer its final child placement does not realize. A correct version
+needs an answer-plus-realization or verification protocol; adding dirtiness
+conditions to the cache does not fix that invariant.
+
 Do not restore the archive's old `DrawMode::Measure`. It skipped primitive and
 retained-state writes while still walking widgets and shaping text, and once
 improved a streamed-frame benchmark from 1.39 to 1.22 ms p50. Retained

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