feat: switch stealth Chromium default channel to consumer Chrome

Playwright's bundled Chromium has a distinct build fingerprint (build ID,
uniform WebGL/codec lists, HeadlessChrome residue) that anti-bot services
increasingly flag. Driving a system-installed Google Chrome via Playwright's
channel option sheds that signal and aligns sec-ch-ua with UA more cleanly.

Changes:
- Add BrowserOptions.Channel string field (chrome, chrome-beta, chromium,
  msedge; empty = default).
- When stealth+headless+Chromium and Channel is empty, default to "chrome"
  (was "chromium"). Explicit Channel values always win, so callers can opt
  back to "chromium" or pick another channel.
- Merge Channel in mergeOptions.
- Expose --channel/--ch flag on cmd/browser for A/B fingerprint testing.

Callers must have the chosen browser installed on the host
(e.g. `playwright install chrome`). Firefox and WebKit paths are untouched.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

browser_init.go

@@ -104,8 +104,17 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		if len(launchArgs) > 0 {
 			launchOpts.Args = launchArgs
 		}
-		if stealth && opt.Browser == BrowserChromium && headless {
-			launchOpts.Channel = playwright.String("chromium")
+		if opt.Browser == BrowserChromium {
+			channel := opt.Channel
+			if channel == "" && stealth && headless {
+				// Real Chrome sheds Playwright's bundled-Chromium fingerprint
+				// (build ID, uniform WebGL, HeadlessChrome residue) that
+				// anti-bot services increasingly flag.
+				channel = "chrome"
+			}
+			if channel != "" {
+				launchOpts.Channel = playwright.String(channel)
+			}
 		}
 		browser, err = bt.Launch(launchOpts)
 		if err != nil {
@@ -216,6 +225,9 @@ func mergeOptions(base BrowserOptions, opts []BrowserOptions) BrowserOptions {
 		if o.Stealth != nil {
 			base.Stealth = o.Stealth
 		}
+		if o.Channel != "" {
+			base.Channel = o.Channel
+		}
 	}
 	return base
 }