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/fixtures/ComfyPage.ts

@@ -24,6 +24,7 @@ import {
 } from './components/SidebarTab'
 import { Topbar } from './components/Topbar'
 import { CanvasHelper } from './helpers/CanvasHelper'
+import { PerformanceHelper } from './helpers/PerformanceHelper'
 import { ClipboardHelper } from './helpers/ClipboardHelper'
 import { CommandHelper } from './helpers/CommandHelper'
 import { DragDropHelper } from './helpers/DragDropHelper'
@@ -185,6 +186,7 @@ export class ComfyPage {
   public readonly dragDrop: DragDropHelper
   public readonly command: CommandHelper
   public readonly bottomPanel: BottomPanel
+  public readonly perf: PerformanceHelper
 
   /** Worker index to test user ID */
   public readonly userIds: string[] = []
@@ -229,6 +231,7 @@ export class ComfyPage {
     this.dragDrop = new DragDropHelper(page, this.assetPath.bind(this))
     this.command = new CommandHelper(page)
     this.bottomPanel = new BottomPanel(page)
+    this.perf = new PerformanceHelper(page)
   }
 
   get visibleToasts() {
@@ -436,7 +439,13 @@ export const comfyPageFixture = base.extend<{
     }
 
     await comfyPage.setup()
+
+    const isPerf = testInfo.tags.includes('@perf')
+    if (isPerf) await comfyPage.perf.init()
+
     await use(comfyPage)
+
+    if (isPerf) await comfyPage.perf.dispose()
   },
   comfyMouse: async ({ comfyPage }, use) => {
     const comfyMouse = new ComfyMouse(comfyPage)