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>

playwright.config.ts

@@ -36,7 +36,18 @@ export default defineConfig({
       name: 'chromium',
       use: { ...devices['Desktop Chrome'] },
       timeout: 15000,
-      grepInvert: /@mobile/ // Run all tests except those tagged with @mobile
+      grepInvert: /@mobile|@perf/ // Run all tests except those tagged with @mobile or @perf
+    },
+
+    {
+      name: 'performance',
+      use: {
+        ...devices['Desktop Chrome'],
+        trace: 'retain-on-failure'
+      },
+      timeout: 60_000,
+      grep: /@perf/,
+      fullyParallel: false
     },
 
     {