Add asyncio async file I/O with open_file, AsyncFile, wrap_file, and Path

Adds thread-delegated async file I/O to asyncio, following proven
Trio/anyio patterns. open_file() and AsyncFile wrap synchronous file
objects with async methods via asyncio.to_thread(). Path wraps
pathlib.Path with async I/O methods and sync property passthrough.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kovan

Lib/asyncio/__init__.py

@@ -22,6 +22,7 @@
 from .timeouts import *
 from .threads import *
 from .transports import *
+from .fileio import *
 
 __all__ = (base_events.__all__ +
            coroutines.__all__ +
@@ -39,7 +40,8 @@
            taskgroups.__all__ +
            threads.__all__ +
            timeouts.__all__ +
-           transports.__all__)
+           transports.__all__ +
+           fileio.__all__)
 
 if sys.platform == 'win32':  # pragma: no cover
     from .windows_events import *