Separate layout cost from draw counts

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -144,6 +144,28 @@ now 2,117 cold, 1 for a leaf repaint, 11 for a scroll, and 3,139 for a resize
 five reference renders, resize render, and image-tab replay are byte-identical
 to the prior #18 head.
 
+Those draw counts explain mechanism, not total layout cost. Before #16,
+measurement was a separate operation: the cold and resize rows each made 839
+draws plus 489 size queries, 314 of which hit the size cache. Current sizing is
+a draw, so its draw count includes the provisional work and cannot be compared
+directly with the old draw count. Fresh release wall-time measurements put the
+complete CPU layout cost in perspective:
+
+| per frame | before #16 | `84f589e` |
+| --- | --- | --- |
+| cold layout | 18.9 ms | 20.5–22.0 ms |
+| repaint one leaf | below timer resolution | below timer resolution |
+| scroll one scroller | below timer resolution | 0.002 ms |
+| resize the output | 3.14 ms median | 12.3–13.4 ms median |
+
+Wall time moves with CPU frequency, so retained benchmark counters should
+report CPU cycles or instructions as the deciding total, then separate widget
+draws, size queries/probes, text shapes and primitive writes to explain it. A
+whole-rig `perf stat` run, dominated by 100 resize frames, measured 13.76B
+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.
+
 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
@@ -176,6 +198,14 @@ 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.
 
+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
+placement later replaced it because a provisional draw is usually already
+usable in its final box; measure-only mode would discard that useful output
+and force another real traversal. The owner confirmed on 2026-09-14 that
+direct placement is the intended path.
+
 The remaining costs are:
 
 - **A container measures a child by drawing it in a box it will not keep.**