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/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'");