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/args.ts

@@ -0,0 +1,49 @@
+/**
+ * Shared argument builder for Playwright and Puppeteer wrappers.
+ */
+
+import type { LaunchOptions } from "./types.js";
+import { getDefaultStealthArgs } from "./config.js";
+
+const DEBUG = /\bcloakbrowser\b/.test(process.env.DEBUG ?? "");
+
+/**
+ * Build deduplicated Chromium CLI args from stealth defaults + user overrides.
+ *
+ * Priority: stealth defaults < user args < dedicated params (timezone/locale).
+ */
+export function buildArgs(options: LaunchOptions): string[] {
+  const seen = new Map<string, string>();
+
+  if (options.stealthArgs !== false) {
+    for (const arg of getDefaultStealthArgs()) {
+      seen.set(arg.split("=")[0], arg);
+    }
+  }
+  if (options.args) {
+    for (const arg of options.args) {
+      const key = arg.split("=")[0];
+      if (seen.has(key)) {
+        if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${arg}`);
+      }
+      seen.set(key, arg);
+    }
+  }
+  if (options.timezone) {
+    const key = "--fingerprint-timezone";
+    const flag = `${key}=${options.timezone}`;
+    if (seen.has(key)) {
+      if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${flag}`);
+    }
+    seen.set(key, flag);
+  }
+  if (options.locale) {
+    const key = "--lang";
+    const flag = `${key}=${options.locale}`;
+    if (seen.has(key)) {
+      if (DEBUG) console.debug(`[cloakbrowser] Arg override: ${seen.get(key)} -> ${flag}`);
+    }
+    seen.set(key, flag);
+  }
+  return [...seen.values()];
+}