Author the lower-level Playwright fixture pattern that every Chromium extension test depends on - `chromium.launchPersistentContext` with `--disable-extensions-except=$DIR` + `--load-extension=$DIR`, the `channel: 'chromium'` selection that unlocks headless extension support, the `context.serviceWorkers()` + `waitForEvent('serviceworker')` race-handling pattern, and the `extensionId = serviceWorker.url().split('/')[2]` extraction recipe. This is the launch-and-load layer shared by every extension test, not the assertions run on top of it. Use when authoring or debugging the fixture a Chromium extension test imports - popup, content script, service worker, options page, or side panel.