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>

browser_tests/helpers/perfReporter.ts

@@ -0,0 +1,49 @@
+import { mkdirSync, readdirSync, readFileSync, writeFileSync } from 'fs'
+import { join } from 'path'
+
+import type { PerfMeasurement } from '../fixtures/helpers/PerformanceHelper'
+
+export interface PerfReport {
+  timestamp: string
+  gitSha: string
+  branch: string
+  measurements: PerfMeasurement[]
+}
+
+const TEMP_DIR = join('test-results', 'perf-temp')
+
+export function recordMeasurement(m: PerfMeasurement) {
+  mkdirSync(TEMP_DIR, { recursive: true })
+  const filename = `${m.name}-${Date.now()}.json`
+  writeFileSync(join(TEMP_DIR, filename), JSON.stringify(m))
+}
+
+export function writePerfReport(
+  gitSha = process.env.GITHUB_SHA ?? 'local',
+  branch = process.env.GITHUB_HEAD_REF ?? 'local'
+) {
+  if (!readdirSync('test-results', { withFileTypes: true }).length) return
+
+  let tempFiles: string[]
+  try {
+    tempFiles = readdirSync(TEMP_DIR).filter((f) => f.endsWith('.json'))
+  } catch {
+    return
+  }
+  if (tempFiles.length === 0) return
+
+  const measurements: PerfMeasurement[] = tempFiles.map((f) =>
+    JSON.parse(readFileSync(join(TEMP_DIR, f), 'utf-8'))
+  )
+
+  const report: PerfReport = {
+    timestamp: new Date().toISOString(),
+    gitSha,
+    branch,
+    measurements
+  }
+  writeFileSync(
+    join('test-results', 'perf-metrics.json'),
+    JSON.stringify(report, null, 2)
+  )
+}