test(e2e): add 4 Linux launcher-routing tests for Phase 9

E9-E12 exercise the launcher's Linux code paths without spawning a
real Firefox binary or Xvfb. They monkeypatch ``sys.platform`` and
stub ``make_virtual_display`` / ``_binary_on_path`` so the tests run
on any host:

- E9 ``_build_prefs(headless=True)`` on Linux passes
  ``virtual_display=False`` to the translator, so the Win32-only
  ``security.sandbox.gpu.level`` workaround never leaks into Linux
  prefs (Xvfb handles window hiding instead).
- E10 ``_resolve_headless`` on Linux + headless=True invokes the
  dispatcher and stores the returned object on ``self._virtual_display``.
- E11 ``_teardown`` stops the Linux virtual display, clears the
  reference, and is idempotent on a second call.
- E12 With Xvfb missing from PATH, ``_resolve_headless`` raises a
  clear ``RuntimeError`` mentioning ``Xvfb`` instead of a cryptic
  FileNotFoundError.

Suite on Linux/WSL: 286 passed, 5 skipped (4 binary-gated E2E
lifecycle tests + 1 Win32 ctypes test). Binary-gated E1/E2/E5/E8
remain ready to run on Linux once the patched Firefox tar.gz is
fetched locally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_e2e.py

@@ -141,3 +141,103 @@ def test_e8_new_context_defaults_from_profile(firefox_binary):
             assert vp["height"] > 0
         finally:
             ctx.close()
+
+
+# ────────────────────────────────────────────────────────────────────
+# Linux-specific lifecycle tests (no Firefox binary required).
+#
+# These exercise the launcher's Linux code paths without spawning real
+# Firefox or Xvfb. They monkeypatch ``sys.platform`` and (where needed)
+# the ``make_virtual_display`` dispatcher so the tests run on any host
+# — including Windows hosts that ship the production CI for this repo.
+# ────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.e2e
+def test_e9_linux_build_prefs_omits_windows_sandbox_key(monkeypatch):
+    """E9: ``_build_prefs(headless=True)`` on Linux must pass
+    ``virtual_display=False`` to the prefs translator. The Win32-only
+    ``security.sandbox.gpu.level`` workaround targets the alt-desktop
+    GPU sandbox bug and MUST NOT leak into Linux prefs, where Xvfb
+    handles window hiding instead."""
+    import sys as _sys
+    monkeypatch.setattr(_sys, "platform", "linux")
+    ip = InvisiblePlaywright(seed=42, headless=True)
+    prefs = ip._build_prefs()
+    assert "security.sandbox.gpu.level" not in prefs
+
+
+@pytest.mark.e2e
+def test_e10_linux_resolve_headless_invokes_xvfb_dispatcher(monkeypatch):
+    """E10: ``_resolve_headless`` with ``headless=True`` on Linux must
+    call ``make_virtual_display().start()`` and store the result on
+    ``self._virtual_display``. We stub the dispatcher so no real Xvfb
+    is spawned — the dispatcher's platform routing is covered separately
+    in ``test_headless.py``."""
+    import sys as _sys
+    monkeypatch.setattr(_sys, "platform", "linux")
+
+    events: list[str] = []
+
+    class _FakeDisplay:
+        def start(self) -> None:
+            events.append("start")
+
+        def stop(self) -> None:
+            events.append("stop")
+
+    from invisible_playwright import launcher as _l
+    monkeypatch.setattr(_l, "make_virtual_display", lambda: _FakeDisplay())
+
+    ip = InvisiblePlaywright(seed=42, headless=True)
+    result = ip._resolve_headless()
+    assert result is False
+    assert events == ["start"]
+    assert ip._virtual_display is not None
+
+
+@pytest.mark.e2e
+def test_e11_linux_teardown_stops_virtual_display_and_is_idempotent(monkeypatch):
+    """E11: ``_teardown`` stops the Linux virtual display, clears the
+    reference, and a second invocation is a no-op. Guards the cleanup
+    path used by ``__exit__`` so a failed ``__enter__`` cannot leak Xvfb."""
+    import sys as _sys
+    monkeypatch.setattr(_sys, "platform", "linux")
+
+    stops: list[bool] = []
+
+    class _FakeDisplay:
+        def start(self) -> None:
+            pass
+
+        def stop(self) -> None:
+            stops.append(True)
+
+    from invisible_playwright import launcher as _l
+    monkeypatch.setattr(_l, "make_virtual_display", lambda: _FakeDisplay())
+
+    ip = InvisiblePlaywright(seed=42, headless=True)
+    ip._resolve_headless()
+    ip._teardown()
+    assert stops == [True]
+    assert ip._virtual_display is None
+    ip._teardown()
+    assert stops == [True]
+
+
+@pytest.mark.e2e
+def test_e12_linux_resolve_headless_without_xvfb_raises_clear_error(monkeypatch):
+    """E12: On Linux with ``headless=True`` and ``Xvfb`` missing from
+    ``PATH``, ``_resolve_headless`` must surface a clear, actionable
+    ``RuntimeError`` instead of a cryptic FileNotFoundError. Verifies
+    the early-check path in ``_LinuxVirtualDisplay.start``."""
+    import sys as _sys
+    monkeypatch.setattr(_sys, "platform", "linux")
+
+    from invisible_playwright import _headless as _h
+    monkeypatch.setattr(_h, "_binary_on_path", lambda name: False)
+
+    ip = InvisiblePlaywright(seed=42, headless=True)
+    with pytest.raises(RuntimeError, match="Xvfb"):
+        ip._resolve_headless()
+    assert ip._virtual_display is None