feat: add performance testing infrastructure with CDP metrics (#9170)

## Summary

Add a permanent, non-failing performance regression detection system
using Chrome DevTools Protocol metrics, with automatic PR commenting.

## Changes

- **What**: Performance testing infrastructure — `PerformanceHelper`
fixture class using CDP `Performance.getMetrics` to collect
`RecalcStyleCount`, `LayoutCount`, `LayoutDuration`, `TaskDuration`,
`JSHeapUsedSize`. Adds `@perf` Playwright project (Chromium-only,
single-threaded, 60s timeout), 4 baseline perf tests, CI workflow with
sticky PR comment reporting, and `perf-report.js` script for generating
markdown comparison tables.

## Review Focus

- `PerformanceHelper` uses `page.context().newCDPSession(page)` — CDP is
Chromium-only, so perf metrics are not collected on Firefox. This is
intentional since CDP gives us browser-level style recalc/layout counts
that `performance.mark/measure` cannot capture.
- The CI workflow uses `continue-on-error: true` so perf tests never
block merging.
- Baseline comparison uses `dawidd6/action-download-artifact` to
download metrics from the target branch, following the same pattern as
`pr-size-report.yaml`.

## Stack

This is the foundation PR for the Firefox performance fix stack:
1. **→ This PR: perf testing infrastructure**
2. `perf/fix-cursor-cache` — cursor style caching (depends on this)
3. `perf/fix-subgraph-svg` — SVG pre-rasterization (depends on this)
4. `perf/fix-clippath-raf` — RAF batching for clip-path (depends on
this)

PRs 2-4 are independent of each other.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9170-feat-add-performance-testing-infrastructure-with-CDP-metrics-3116d73d3650817cb43def6f8e9917f8)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

scripts/perf-report.ts

@@ -0,0 +1,125 @@
+import { existsSync, readFileSync } from 'node:fs'
+
+interface PerfMeasurement {
+  name: string
+  durationMs: number
+  styleRecalcs: number
+  styleRecalcDurationMs: number
+  layouts: number
+  layoutDurationMs: number
+  taskDurationMs: number
+  heapDeltaBytes: number
+}
+
+interface PerfReport {
+  timestamp: string
+  gitSha: string
+  branch: string
+  measurements: PerfMeasurement[]
+}
+
+const CURRENT_PATH = 'test-results/perf-metrics.json'
+const BASELINE_PATH = 'temp/perf-baseline/perf-metrics.json'
+
+function formatDelta(pct: number): string {
+  if (pct >= 20) return `+${pct.toFixed(0)}% 🔴`
+  if (pct >= 10) return `+${pct.toFixed(0)}% 🟠`
+  if (pct > -10) return `${pct >= 0 ? '+' : ''}${pct.toFixed(0)}% ⚪`
+  return `${pct.toFixed(0)}% 🟢`
+}
+
+function formatBytes(bytes: number): string {
+  if (Math.abs(bytes) < 1024) return `${bytes} B`
+  if (Math.abs(bytes) < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`
+}
+
+function calcDelta(
+  baseline: number,
+  current: number
+): { pct: number; isNew: boolean } {
+  if (baseline > 0) {
+    return { pct: ((current - baseline) / baseline) * 100, isNew: false }
+  }
+  return current > 0 ? { pct: Infinity, isNew: true } : { pct: 0, isNew: false }
+}
+
+function formatDeltaCell(delta: { pct: number; isNew: boolean }): string {
+  return delta.isNew ? 'new 🔴' : formatDelta(delta.pct)
+}
+
+function main() {
+  if (!existsSync(CURRENT_PATH)) {
+    process.stdout.write(
+      '## ⚡ Performance Report\n\nNo perf metrics found. Perf tests may not have run.\n'
+    )
+    process.exit(0)
+  }
+
+  const current: PerfReport = JSON.parse(readFileSync(CURRENT_PATH, 'utf-8'))
+
+  const baseline: PerfReport | null = existsSync(BASELINE_PATH)
+    ? JSON.parse(readFileSync(BASELINE_PATH, 'utf-8'))
+    : null
+
+  const lines: string[] = []
+  lines.push('## ⚡ Performance Report\n')
+
+  if (baseline) {
+    lines.push(
+      '| Metric | Baseline | PR | Δ |',
+      '|--------|----------|-----|---|'
+    )
+
+    for (const m of current.measurements) {
+      const base = baseline.measurements.find((b) => b.name === m.name)
+      if (!base) {
+        lines.push(`| ${m.name}: style recalcs | — | ${m.styleRecalcs} | new |`)
+        lines.push(`| ${m.name}: layouts | — | ${m.layouts} | new |`)
+        lines.push(
+          `| ${m.name}: task duration | — | ${m.taskDurationMs.toFixed(0)}ms | new |`
+        )
+        continue
+      }
+
+      const recalcDelta = calcDelta(base.styleRecalcs, m.styleRecalcs)
+      lines.push(
+        `| ${m.name}: style recalcs | ${base.styleRecalcs} | ${m.styleRecalcs} | ${formatDeltaCell(recalcDelta)} |`
+      )
+
+      const layoutDelta = calcDelta(base.layouts, m.layouts)
+      lines.push(
+        `| ${m.name}: layouts | ${base.layouts} | ${m.layouts} | ${formatDeltaCell(layoutDelta)} |`
+      )
+
+      const taskDelta = calcDelta(base.taskDurationMs, m.taskDurationMs)
+      lines.push(
+        `| ${m.name}: task duration | ${base.taskDurationMs.toFixed(0)}ms | ${m.taskDurationMs.toFixed(0)}ms | ${formatDeltaCell(taskDelta)} |`
+      )
+    }
+  } else {
+    lines.push(
+      'No baseline found — showing absolute values.\n',
+      '| Metric | Value |',
+      '|--------|-------|'
+    )
+    for (const m of current.measurements) {
+      lines.push(`| ${m.name}: style recalcs | ${m.styleRecalcs} |`)
+      lines.push(`| ${m.name}: layouts | ${m.layouts} |`)
+      lines.push(
+        `| ${m.name}: task duration | ${m.taskDurationMs.toFixed(0)}ms |`
+      )
+      lines.push(`| ${m.name}: heap delta | ${formatBytes(m.heapDeltaBytes)} |`)
+    }
+  }
+
+  lines.push('\n<details><summary>Raw data</summary>\n')
+  lines.push('```json')
+  lines.push(JSON.stringify(current, null, 2))
+  lines.push('```')
+  lines.push('\n</details>')
+
+  process.stdout.write(lines.join('\n') + '\n')
+}
+
+main()