Document retained layout diagnostics

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 `84f589e`, fifteen commits,
+worktree `/home/bob/repos/iris-pr18`, head `480f0bc`, sixteen 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.
 
@@ -166,6 +166,35 @@ instructions current versus 3.56B before #16 (3.87x), consistent with the
 resize wall-time gap. A follow-up rig should select one phase per invocation so
 the hardware counters are phase-specific rather than inferred from that total.
 
+`480f0bc` retains that instrumentation behind the `layout-diagnostics` Cargo
+feature; none of it is compiled into a normal Iris build. The ignored
+`tests/layout_diagnostics.rs` rig selects `cold`, `repaint`, `size`, `scroll`,
+or `resize` with `IRIS_PHASE`, plus seed, depth and frame count. Run it with the
+feature for explanatory counters and inclusive phase timers, and without the
+feature under `perf` for unperturbed CPU totals:
+
+```sh
+IRIS_PHASE=resize IRIS_DEPTH=8 IRIS_FRAMES=100 \
+  cargo test --release --features layout-diagnostics \
+  --test layout_diagnostics -- --ignored --nocapture
+
+IRIS_PHASE=resize IRIS_DEPTH=8 IRIS_FRAMES=1000 \
+  perf stat -e cycles:u,instructions:u cargo test --release \
+  --test layout_diagnostics -- --ignored --nocapture
+```
+
+The first depth-8 resize run makes the amplification concrete. A 260-widget
+tree has 215 active widgets, but a resize averages 1,395 widget draws, 913
+placement calls, 1,313 reads of drawn sizes and 34,844 primitive writes. Only
+12 distinct text widgets render, yet they render and reshape 282 times per
+frame with no shape-cache hits. Text rendering accounts for 11.6 of 13.2 ms,
+including 9.9 ms shaping and 1.7 ms placing glyphs. The hottest two text
+widgets each draw 96 times per frame at 22 distinct widths across two resize
+frames, below nested `Span`, `Aligned`, `Scroll`, `Pad`, and `SetSize`
+ancestors. This identifies repeated constraint discovery, rather than resize
+marking (0.001 ms), as the next subject; it does not yet choose between
+per-axis retained size validity and coalescing the resize dependency frontier.
+
 The generated tree now includes `Aligned` with every meaningful per-axis
 alignment. That exposed two retained-layout ordering bugs which `84f589e`
 fixes. The regular cold-layout equivalence suite passes. The ignored 100-seed