feat: add statistical significance to perf report with z-score thresholds (#9305)

## Summary

Replace fixed 10%/20% perf delta thresholds with dynamic σ-based
classification using z-scores, eliminating false alarms from naturally
noisy duration metrics (10-17% CV).

## Changes

- **What**:
- Run each perf test 3× (`--repeat-each=3`) and report the mean,
reducing single-run noise
- Download last 5 successful main branch perf artifacts to compute
historical μ/σ per metric
- Replace fixed threshold flags with z-score significance: `⚠️
regression` (z>2), `✅ neutral/improvement`, `🔇 noisy` (CV>50%)
  - Add collapsible historical variance table (μ, σ, CV) to PR comment
- Graceful cold start: falls back to simple delta table until ≥2
historical runs exist
- New `scripts/perf-stats.ts` module with `computeStats`, `zScore`,
`classifyChange`
  - 18 unit tests for stats functions

- **CI time impact**: ~3 min → ~5-6 min (repeat-each adds ~2 min,
historical download <10s)

## Review Focus

- The `gh api` call in the new "Download historical perf baselines"
step: it queries the last 5 successful push runs on the base branch. The
`gh` CLI is available natively on `ubuntu-latest` runners and
auto-authenticates with `GITHUB_TOKEN`.
- `getHistoricalStats` averages per-run measurements before computing
cross-run σ — this is intentional since historical artifacts may also
contain repeated measurements after this change lands.
- The `noisy` classification (CV>50%) suppresses metrics like `layouts`
that hover near 0 and have meaningless percentage swings.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9305-feat-add-statistical-significance-to-perf-report-with-z-score-thresholds-3156d73d3650818d9360eeafd9ae7dc1)
by [Unito](https://www.unito.io)

.github/workflows/ci-perf-report.yaml

@@ -45,7 +45,7 @@ jobs:
       - name: Run performance tests
         id: perf
         continue-on-error: true
-        run: pnpm exec playwright test --project=performance --workers=1
+        run: pnpm exec playwright test --project=performance --workers=1 --repeat-each=3
 
       - name: Upload perf metrics
         if: always()
@@ -61,6 +61,7 @@ jobs:
     if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
     permissions:
+      actions: read
       contents: read
       pull-requests: write
 
@@ -90,6 +91,31 @@ jobs:
           path: temp/perf-baseline/
           if_no_artifact_found: warn
 
+      - name: Download historical perf baselines
+        continue-on-error: true
+        run: |
+          RUNS=$(gh api \
+            "/repos/${{ github.repository }}/actions/workflows/ci-perf-report.yaml/runs?branch=${{ github.event.pull_request.base.ref }}&event=push&status=success&per_page=5" \
+            --jq '.workflow_runs[].id' || true)
+
+          if [ -z "$RUNS" ]; then
+            echo "No historical runs available"
+            exit 0
+          fi
+
+          mkdir -p temp/perf-history
+          INDEX=0
+          for RUN_ID in $RUNS; do
+            DIR="temp/perf-history/$INDEX"
+            mkdir -p "$DIR"
+            gh run download "$RUN_ID" -n perf-metrics -D "$DIR/" 2>/dev/null || true
+            INDEX=$((INDEX + 1))
+          done
+
+          echo "Downloaded $(ls temp/perf-history/*/perf-metrics.json 2>/dev/null | wc -l) historical baselines"
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Generate perf report
         run: npx --yes tsx scripts/perf-report.ts > perf-report.md