Record PR 19 performance sweep and request invalidation fix

docs/LAYOUT.md

@@ -66,10 +66,24 @@ The allocation rig forces every widget to redraw across 100 resize frames:
 both the plain and clamped eight-row fixtures allocate zero times after warm-up.
 This is a measured property of those stable visible trees, not a guarantee for
 arbitrary widgets or visibility changes. An eight-level known nested span paints
-its leaf once. The 40-row, 1,000-frame text resize rig costs 9,891,085,554 retired
-instructions versus 9,341,015,256 at `de1eb7e` (5.89% more), with the same printed
-geometry. Request dependency tracking has a cost even when text still needs
-measurement. The 400-tree unbounded cold dump preserves all 34,986 boxes.
+its leaf once. Discovery retains dependencies only for requests it actually
+uses; discarded discovery leaves dependency tracking to the measured draw.
+Used hints still register request readers so a newly added cap reallocates the
+parent even when the child's reported share is unchanged.
+
+The performance sweep of 2026-09-20 found and removed an eightfold increase in
+the unchanged-paragraph repaint fixture caused by discarded discovery
+invalidating its span. The 40-row, 1,000-frame text resize rig now costs 9.432B
+retired instructions against 9.340B at `de1eb7e` (+0.99%), down from 9.890B
+at `8780b40` (+5.90%). Text draw, line-break and glyph-placement counts agree;
+the remaining cost is layout bookkeeping. The width sweep is +1.23%, and
+paragraph edits and scrolling are within 0.3% in the measured fixtures.
+These are medians of nine process totals, not GPU or phone frame times.
+Deep generated trees still have workload-dependent wins and regressions;
+`docs/LAYOUT_LOG.md` records the matrix, storage costs and measurement limits.
+The 400-tree unbounded cold dump preserves all 34,986 boxes against `de1eb7e`;
+the performance correction also preserves all 34,986 bounded boxes against
+`8780b40`.
 
 The app's submodule pin is unchanged. This design lives on Iris PR #19.