Skip transactor lifecycle tests where AF_UNIX is unavailable

The transactor's client and server speak over AF_UNIX, which the client
already treats as POSIX-only. These tests opened an AF_UNIX listener
directly and failed on the Windows CI legs (Python 3.13 and 3.14).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Rusty Conover

tests/transactor/test_completion_lifecycle.py

@@ -28,6 +28,11 @@
 from vgi.transactor.protocol import TransactorProtocol
 from vgi.transactor.server import TransactorImpl
 
+# The transactor's client and server speak over AF_UNIX, which is POSIX-only.
+pytestmark = pytest.mark.skipif(
+    not hasattr(socket, "AF_UNIX"), reason="the transactor transport is AF_UNIX (POSIX-only)"
+)
+
 
 @pytest.fixture
 def transactor(tmp_path: Path) -> Iterator[tuple[TransactorImpl, bytes]]: