feat: buildGStackLaunchArgs — Pack 1 cmdline-switch construction for gbrowser

New stealth.ts export that turns the GSTACK_* env vars (already populated
by gbrowser's gbd from host_profile.go) into the --gstack-* cmdline
switches the Pack 1 Chromium patches read at WebGL getParameter,
NavigatorUA::userAgentData, NavigatorConcurrentHardware::hardwareConcurrency,
and NavigatorDeviceMemory::deviceMemory time.

Wired into all three launchArgs sites: launch() (headless), launchHeaded()
(real product path), and handoff() (headless → headed re-launch).

Mapping:
  GSTACK_GPU_VENDOR      → --gstack-gpu-vendor
  GSTACK_GPU_RENDERER    → --gstack-gpu-renderer
  GSTACK_PLATFORM        → --gstack-ua-platform (with mapping:
                            MacARM/MacIntel → macOS, Win32 → Windows,
                            Linux x86_64 → Linux)
  GSTACK_GPU_CHIPSET     → --gstack-ua-model
  GSTACK_HW_CONCURRENCY  → --gstack-hw-concurrency
  GSTACK_DEVICE_MEMORY   → --gstack-device-memory

Each switch is emitted only when its env var is non-empty — empty
values fall through to the patch's "no override" path, which returns
the real Chromium native value. Safe to ship on Chromium builds
without the Pack 1 patches applied (zero behavior change).

The patches themselves live in the gbrowser repo at chromium/patches/
{webgl-vendor-spoof,ua-client-hints-stealth,worker-navigator-stealth}.patch.
Both halves (gstack arg construction + gbrowser C++ patches) must
land + Chromium rebuild before the spoof reaches the WebGL/UA-CH/
hardware accessors. Currently dormant until then.

Tests (browse/test/stealth-layer-c.test.ts):
  7 new buildGStackLaunchArgs cases — empty env, all-populated, partial,
  platform mapping (MacARM/MacIntel/Win32/Linux), unrecognized platform
  fallthrough, vendor-with-spaces escape-safety.
  All 32 stealth/browser-manager tests pass.

For GBrowser specifically: gstack-side half of the Pack 1 flag plumbing.
gbrowser repo will bump the submodule pointer to this commit, then re-run
bun run test/anti-bot/evidence-run.ts to verify creepjs's "33% headless"
score drops after Pack 1 + Chromium rebuild.

browse/src/browser-manager.ts

@@ -213,8 +213,8 @@ export class BrowserManager {
     // BROWSE_EXTENSIONS_DIR points to an unpacked Chrome extension directory.
     // Extensions only work in headed mode, so we use an off-screen window.
     const extensionsDir = process.env.BROWSE_EXTENSIONS_DIR;
-    const { STEALTH_LAUNCH_ARGS } = await import('./stealth');
-    const launchArgs: string[] = [...STEALTH_LAUNCH_ARGS];
+    const { STEALTH_LAUNCH_ARGS, buildGStackLaunchArgs } = await import('./stealth');
+    const launchArgs: string[] = [...STEALTH_LAUNCH_ARGS, ...buildGStackLaunchArgs()];
     let useHeadless = true;
 
     // Docker/CI/root: Chromium sandbox requires unprivileged user namespaces which
@@ -295,11 +295,18 @@ export class BrowserManager {
 
     // Find the gstack extension directory for auto-loading
     const extensionPath = this.findExtensionPath();
+    const { buildGStackLaunchArgs } = await import('./stealth');
     const launchArgs = [
       '--hide-crash-restore-bubble',
       // Anti-bot-detection: remove the navigator.webdriver flag that Playwright sets.
       // Sites like Google and NYTimes check this to block automation browsers.
       '--disable-blink-features=AutomationControlled',
+      // GStack Pack 1: per-install hardware/GPU/UA-CH overrides for the
+      // C++ patches in gbrowser's Chromium build. Each switch is a no-op
+      // on Chromium builds without the corresponding patch (the patch's
+      // empty-fallback returns native), so this is safe on stock Playwright
+      // Chromium too.
+      ...buildGStackLaunchArgs(),
     ];
     if (extensionPath) {
       // Skip --load-extension when running against a custom Chromium build
@@ -1296,7 +1303,8 @@ export class BrowserManager {
       const fs = require('fs');
       const path = require('path');
       const extensionPath = this.findExtensionPath();
-      const launchArgs = ['--hide-crash-restore-bubble'];
+      const { buildGStackLaunchArgs } = await import('./stealth');
+      const launchArgs: string[] = ['--hide-crash-restore-bubble', ...buildGStackLaunchArgs()];
       if (extensionPath) {
         launchArgs.push(`--disable-extensions-except=${extensionPath}`);
         launchArgs.push(`--load-extension=${extensionPath}`);

browse/src/stealth.ts

@@ -270,6 +270,62 @@ export const STEALTH_LAUNCH_ARGS = [
   '--disable-blink-features=AutomationControlled',
 ];
 
+/**
+ * Build the `--gstack-*=` cmdline switches that the Pack 1 Chromium
+ * patches read (webgl-vendor-spoof, ua-client-hints-stealth, worker-
+ * navigator-stealth). Values come from the GSTACK_* env vars that
+ * gbd populates from host_profile.go at startup.
+ *
+ * Each switch is only emitted when its env var is non-empty — empty
+ * env values fall through to the patch's "no override" path, which
+ * returns the real Chromium native value. This keeps the helper safe
+ * on builds that DO NOT have the C++ patches applied (gbrowser
+ * pre-Pack-1) and on hosts where gbd hasn't yet populated some
+ * fields (legacy installs).
+ *
+ * Mapping (gbd env → Chromium cmdline switch → C++ patch consumer):
+ *   GSTACK_GPU_VENDOR        → --gstack-gpu-vendor        → webgl-vendor-spoof.patch
+ *   GSTACK_GPU_RENDERER      → --gstack-gpu-renderer      → webgl-vendor-spoof.patch
+ *   GSTACK_PLATFORM          → --gstack-ua-platform       → ua-client-hints-stealth.patch
+ *                              (maps MacARM/MacIntel → "macOS")
+ *   GSTACK_GPU_CHIPSET       → --gstack-ua-model          → ua-client-hints-stealth.patch
+ *   GSTACK_HW_CONCURRENCY    → --gstack-hw-concurrency    → worker-navigator-stealth.patch
+ *   GSTACK_DEVICE_MEMORY     → --gstack-device-memory     → worker-navigator-stealth.patch
+ */
+export function buildGStackLaunchArgs(): string[] {
+  const env = (globalThis as any).process?.env ?? {};
+  const args: string[] = [];
+
+  const vendor = env.GSTACK_GPU_VENDOR;
+  if (vendor) args.push(`--gstack-gpu-vendor=${vendor}`);
+
+  const renderer = env.GSTACK_GPU_RENDERER;
+  if (renderer) args.push(`--gstack-gpu-renderer=${renderer}`);
+
+  // Map gbd's "MacARM"/"MacIntel" classification to the UA-CH "macOS"
+  // platform string Chromium emits natively. Other future platforms
+  // would map similarly (Win32 → "Windows", Linux → "Linux").
+  const platform = env.GSTACK_PLATFORM;
+  if (platform === 'MacARM' || platform === 'MacIntel') {
+    args.push('--gstack-ua-platform=macOS');
+  } else if (platform === 'Win32') {
+    args.push('--gstack-ua-platform=Windows');
+  } else if (platform && platform.startsWith('Linux')) {
+    args.push('--gstack-ua-platform=Linux');
+  }
+
+  const chipset = env.GSTACK_GPU_CHIPSET;
+  if (chipset) args.push(`--gstack-ua-model=${chipset}`);
+
+  const hw = env.GSTACK_HW_CONCURRENCY;
+  if (hw) args.push(`--gstack-hw-concurrency=${hw}`);
+
+  const memory = env.GSTACK_DEVICE_MEMORY;
+  if (memory) args.push(`--gstack-device-memory=${memory}`);
+
+  return args;
+}
+
 /**
  * Playwright default args to strip via ignoreDefaultArgs.
  *

browse/test/stealth-layer-c.test.ts

@@ -11,6 +11,7 @@
 import { describe, test, expect } from 'bun:test';
 import {
   buildStealthScript,
+  buildGStackLaunchArgs,
   WEBDRIVER_MASK_SCRIPT,
   STEALTH_LAUNCH_ARGS,
   STEALTH_IGNORE_DEFAULT_ARGS,
@@ -116,6 +117,97 @@ describe('buildStealthScript — T3 Layer C', () => {
   });
 });
 
+describe('buildGStackLaunchArgs — Pack 1 cmdline-switch construction', () => {
+  // Helper: clear all GSTACK_* env, run test body, restore env.
+  function withEnv(env: Record<string, string | undefined>, body: () => void) {
+    const saved: Record<string, string | undefined> = {};
+    for (const k of Object.keys(process.env)) {
+      if (k.startsWith('GSTACK_')) {
+        saved[k] = process.env[k];
+        delete process.env[k];
+      }
+    }
+    for (const [k, v] of Object.entries(env)) {
+      if (v !== undefined) process.env[k] = v;
+    }
+    try {
+      body();
+    } finally {
+      for (const k of Object.keys(process.env)) {
+        if (k.startsWith('GSTACK_')) delete process.env[k];
+      }
+      for (const [k, v] of Object.entries(saved)) {
+        if (v !== undefined) process.env[k] = v;
+      }
+    }
+  }
+
+  test('empty env produces empty arg list', () => {
+    withEnv({}, () => {
+      expect(buildGStackLaunchArgs()).toEqual([]);
+    });
+  });
+
+  test('all env values populated → all 6 switches emitted', () => {
+    withEnv({
+      GSTACK_GPU_VENDOR: 'Apple Inc.',
+      GSTACK_GPU_RENDERER: 'ANGLE (Apple, ANGLE Metal Renderer: Apple M4 Max, Unspecified Version)',
+      GSTACK_PLATFORM: 'MacARM',
+      GSTACK_GPU_CHIPSET: 'Apple M4 Max',
+      GSTACK_HW_CONCURRENCY: '16',
+      GSTACK_DEVICE_MEMORY: '8',
+    }, () => {
+      const args = buildGStackLaunchArgs();
+      expect(args).toContain('--gstack-gpu-vendor=Apple Inc.');
+      expect(args).toContain('--gstack-gpu-renderer=ANGLE (Apple, ANGLE Metal Renderer: Apple M4 Max, Unspecified Version)');
+      expect(args).toContain('--gstack-ua-platform=macOS');
+      expect(args).toContain('--gstack-ua-model=Apple M4 Max');
+      expect(args).toContain('--gstack-hw-concurrency=16');
+      expect(args).toContain('--gstack-device-memory=8');
+      expect(args.length).toBe(6);
+    });
+  });
+
+  test('platform mapping: MacARM and MacIntel both → macOS', () => {
+    withEnv({ GSTACK_PLATFORM: 'MacARM' }, () => {
+      expect(buildGStackLaunchArgs()).toContain('--gstack-ua-platform=macOS');
+    });
+    withEnv({ GSTACK_PLATFORM: 'MacIntel' }, () => {
+      expect(buildGStackLaunchArgs()).toContain('--gstack-ua-platform=macOS');
+    });
+  });
+
+  test('platform mapping: Win32 → Windows, Linux x86_64 → Linux', () => {
+    withEnv({ GSTACK_PLATFORM: 'Win32' }, () => {
+      expect(buildGStackLaunchArgs()).toContain('--gstack-ua-platform=Windows');
+    });
+    withEnv({ GSTACK_PLATFORM: 'Linux x86_64' }, () => {
+      expect(buildGStackLaunchArgs()).toContain('--gstack-ua-platform=Linux');
+    });
+  });
+
+  test('partial env: only set switches that have values', () => {
+    withEnv({ GSTACK_HW_CONCURRENCY: '12' }, () => {
+      const args = buildGStackLaunchArgs();
+      expect(args).toEqual(['--gstack-hw-concurrency=12']);
+    });
+  });
+
+  test('unrecognized platform falls through without --gstack-ua-platform', () => {
+    withEnv({ GSTACK_PLATFORM: 'OS/2' }, () => {
+      const args = buildGStackLaunchArgs();
+      expect(args.some(a => a.startsWith('--gstack-ua-platform='))).toBe(false);
+    });
+  });
+
+  test('GPU vendor with spaces survives intact (no quote/escape regression)', () => {
+    withEnv({ GSTACK_GPU_VENDOR: 'NVIDIA Corporation' }, () => {
+      const args = buildGStackLaunchArgs();
+      expect(args).toContain('--gstack-gpu-vendor=NVIDIA Corporation');
+    });
+  });
+});
+
 describe('backwards-compat exports', () => {
   test('WEBDRIVER_MASK_SCRIPT still exported', () => {
     expect(WEBDRIVER_MASK_SCRIPT).toContain("'webdriver'");