fix: deduplicate CLI flags when user args overlap with stealth defaults

Bump version to 0.3.9. Extract shared buildArgs into js/src/args.ts (DRY),
guard console.debug behind DEBUG=cloakbrowser env var, strengthen caplog assertion.

js/src/playwright.ts

@@ -5,7 +5,8 @@
 
 import type { Browser, BrowserContext } from "playwright-core";
 import type { LaunchOptions, LaunchContextOptions, LaunchPersistentContextOptions } from "./types.js";
-import { DEFAULT_VIEWPORT, getDefaultStealthArgs } from "./config.js";
+import { DEFAULT_VIEWPORT } from "./config.js";
+import { buildArgs } from "./args.js";
 import { ensureBinary } from "./download.js";
 import { parseProxyUrl } from "./proxy.js";
 
@@ -176,24 +177,4 @@ async function maybeResolveGeoip(
 }
 
 /** @internal Exposed for unit tests only. */
-export function _buildArgsForTest(options: LaunchOptions): string[] {
-  return buildArgs(options);
-}
-
-function buildArgs(options: LaunchOptions): string[] {
-  const args: string[] = [];
-  if (options.stealthArgs !== false) {
-    args.push(...getDefaultStealthArgs());
-  }
-  if (options.args) {
-    args.push(...options.args);
-  }
-  // Timezone/locale flags — always inject when set
-  if (options.timezone) {
-    args.push(`--fingerprint-timezone=${options.timezone}`);
-  }
-  if (options.locale) {
-    args.push(`--lang=${options.locale}`);
-  }
-  return args;
-}
+export { buildArgs as _buildArgsForTest } from "./args.js";