gh-145342: Make the guest I/O thread non-daemonic with graceful shutdown

Address review feedback on the guest-mode runtime:

- The I/O thread is no longer a daemon thread; it is joined when the
  run finishes, and a threading._register_atexit() hook (the
  concurrent.futures pattern) wakes it out of its selector wait so an
  unfinished run cannot hang interpreter shutdown.
- Reuse loop._run_forever_setup()/_run_forever_cleanup() so the whole
  guest run counts as running: get_running_loop() works, is_running()
  is true, nested run_forever()/run_until_complete() raise, asyncgen
  hooks and coroutine origin tracking are installed and restored.  On
  Windows this also establishes the proactor self-reading loop needed
  for call_soon_threadsafe() to wake IocpProactor.select().
- Preserve the host's signal wakeup fd across loop creation and close
  (the proactor installs/resets it in __init__/close on the main
  thread).
- On completion, run the same cleanup as asyncio.run() -- cancel
  remaining tasks, shutdown asyncgens and the default executor, close
  the loop -- before invoking done_callback.
- Harden the host/IO-thread handshake: exception-safe token handoff,
  and an abort path that unwinds the main task if the I/O thread dies.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: congzhangzh