asyncio: Add guest mode for running asyncio inside external event loops

Feature request

Add guest mode to asyncio, enabling it to run cooperatively inside a host event loop such as Tkinter, Qt, GTK, or any other framework that owns the main thread.

Motivation

GUI toolkits (Tk, Qt, GTK, pygame, ...) each have their own event loop that must run on the main thread. Today there is no standard way to use asyncio coroutines inside a GUI application without:

• Polling hacks (periodic root.after() calls)
• Replacing the event loop entirely (fragile, framework-specific)
• Running asyncio in a separate thread and manually marshalling results

Trio solved this problem with trio.lowlevel.start_guest_run() and the approach has proven robust across many GUI frameworks (see trio-guest).

Proposed API

import asyncio

task = asyncio.start_guest_run(
    async_fn, *args,
    run_sync_soon_threadsafe=host.schedule_callback,
    done_callback=host.on_done,
)

The implementation decomposes BaseEventLoop._run_once() into three new public methods — poll_events(), process_events(), and process_ready() — then uses a dual-thread architecture where the host thread processes callbacks and a daemon I/O thread polls for events.

Prior art and discussion

• Trio guest mode
• trio-guest — multi-framework reference implementation
• asyncio-guest — proof-of-concept for asyncio (this proposal is derived from it)
• discuss.python.org: Connecting asyncio and tkinter event loops

A reference implementation with tests (12 tests passing, full asyncio test suite regression-free) is ready to submit as a PR.

Linked PRs

• gh-145343

从 BaseEventLoop._run_once()、提议的公共 API 以及参考实现的 12 个测试开始。审查关联的 PR gh-145343 和引用的访客模式先例；完成的标准是，提议的访客模式能够与宿主事件循环协同工作，并通过其测试，且不会在 asyncio 测试套件中引入回归。