perf: add layout/GC metrics + reduce false positives in regression detection (#10477)

## Summary

Add layout duration, style recalc duration, and heap usage metrics to CI
perf reports, while improving statistical reliability to reduce false
positive regressions.

## Changes

- **What**:
- Collect `layoutDurationMs`, `styleRecalcDurationMs`, `heapUsedBytes`
(absolute snapshot) alongside existing metrics
- Add effect size gate (`minAbsDelta`) for integer-quantized count
metrics (style recalcs, layouts, DOM nodes, event listeners) — prevents
z=7.2 false positives from e.g. 11→12 style recalcs
- Switch from mean to **median** for PR metric aggregation — robust to
outlier CI runs that dominate n=3 mean
- Increase historical baseline window from **5 to 15 runs** for more
stable σ estimates
- Reorder reported metrics: layout/style duration first (actionable),
counts and heap after (informational)

## Review Focus

The effect size gate in `classifyChange()` — it now requires both z > 2
AND absolute delta ≥ `minAbsDelta` (when configured) to flag a
regression. This addresses the core false positive issue where integer
metrics with near-zero historical variance produce extreme z-scores for
trivial changes.

Median vs mean tradeoff: median is more robust to outliers but less
sensitive to real shifts — acceptable given n=3 and CI noise levels.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10477-perf-add-layout-GC-metrics-reduce-false-positives-in-regression-detection-32d6d73d365081daa72cec96d8a07b90)
by [Unito](https://www.unito.io)

.github/workflows/pr-report.yaml

@@ -180,7 +180,7 @@ jobs:
           if git ls-remote --exit-code origin perf-data >/dev/null 2>&1; then
             git fetch origin perf-data --depth=1
             mkdir -p temp/perf-history
-            for file in $(git ls-tree --name-only origin/perf-data baselines/ 2>/dev/null | sort -r | head -10); do
+            for file in $(git ls-tree --name-only origin/perf-data baselines/ 2>/dev/null | sort -r | head -15); do
               git show "origin/perf-data:${file}" > "temp/perf-history/$(basename "$file")" 2>/dev/null || true
             done
             echo "Loaded $(ls temp/perf-history/*.json 2>/dev/null | wc -l) historical baselines"