feat: add trend visualization with sparklines to perf report (#9939)

## Summary

Add historical trend visualization (ASCII sparklines + directional
arrows) to the performance PR report, showing how each metric has moved
over recent commits on main.

## Changes

- **What**: New `sparkline()`, `trendDirection()`, `trendArrow()`
functions in `perf-stats.ts`. New collapsible "Trend" section in the
perf report showing per-metric sparklines, direction indicators, and
latest values. CI workflow updated to download historical data from the
`perf-data` orphan branch and switched to `setup-frontend` action with
`pnpm exec tsx`.

## Review Focus

- The trend section only renders when ≥3 historical data points exist
(gracefully absent otherwise)
- `trendDirection()` uses a split-half mean comparison with ±10%
threshold — review whether this sensitivity is appropriate
- The `git archive` step in `pr-perf-report.yaml` is idempotent and
fails silently if no perf-history data exists yet on the perf-data
branch

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9939-feat-add-trend-visualization-with-sparklines-to-perf-report-3246d73d36508125a6fcc39612f850fe)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

.github/workflows/pr-report.yaml

@@ -173,6 +173,19 @@ jobs:
           path: temp/perf-baseline/
           if_no_artifact_found: warn
 
+      - name: Download perf history from perf-data branch
+        if: steps.pr-meta.outputs.skip != 'true' && steps.find-perf.outputs.status == 'ready'
+        continue-on-error: true
+        run: |
+          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 $(git ls-tree --name-only origin/perf-data baselines/ 2>/dev/null | sort -r | head -10); do
+              git show "origin/perf-data:${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"
+          fi
+
       - name: Generate unified report
         if: steps.pr-meta.outputs.skip != 'true'
         run: >