add opt-in asyncio task integration

docs/plugin-api-additions/async.md

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+# Optional Python asyncio integration
+
+`run_async(coroutine, context=None, interval=10)` returns an AsyncTask with
+`done()`, `cancel()` and `result()`. Each invocation owns a separate event loop
+pumped by an existing main-loop timer. A ready stop callback ensures selector
+polling never blocks the UI. The captured context is restored after every pump;
+a closed context cancels the work. No process-wide event-loop policy is changed.
+
+Existing synchronous scripts are unaffected. Use `asyncio.get_running_loop()`
+inside the coroutine. Child tasks use the same loop/context and are cancelled
+when the main coroutine finishes or the script unloads. Cancellation cleanup is
+bounded; tasks must not suppress cancellation indefinitely. Cleanup uses the
+captured context while it remains open; after it closes, cancellation cleanup uses
+the current valid context. Use explicit Context methods for context-specific
+output in finally blocks, and do not issue context-dependent commands there. This provides normal
+asyncio sockets/timers, not Windows subprocess support or a new HTTP library.
+Asyncio hostname resolution can use its executor; use a resolver appropriate to
+your library if you require no worker threads. Blocking work inside a coroutine
+still blocks the client and must be avoided.
+
+```python
+import asyncio
+import zoitechat
+
+async def later():
+    await asyncio.sleep(1)
+    zoitechat.prnt('Still in the context where this task was started')
+
+task = zoitechat.run_async(later())
+```

plugins/python/_zoitechat_async.py

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+"""Opt-in asyncio tasks pumped without blocking the client main loop."""
+import asyncio
+import operator
+import traceback
+import _zoitechat as api
+
+__all__ = ['AsyncTask', 'run_async']
+
+
+class AsyncTask:
+    """A script-owned task and isolated event loop; cancel() is cooperative."""
+    def __init__(self, coroutine, plugin, context, interval):
+        self._plugin = plugin
+        self._context = context
+        self._loop = asyncio.new_event_loop()
+        self._task = self._loop.create_task(coroutine)
+        self._closed = False
+        self._unload_hook = api.hook_unload(lambda data: self._close())
+        self._timer_hook = api.hook_timer(interval, self._tick)
+
+    def done(self):
+        return self._task.done()
+
+    def cancel(self):
+        if not self._closed:
+            return self._task.cancel()
+        return False
+
+    def result(self):
+        return self._task.result()
+
+    def _pump(self):
+        # A ready stop callback forces the selector's timeout to zero.
+        self._loop.call_soon(self._loop.stop)
+        self._loop.run_forever()
+
+    def _close(self):
+        if self._closed:
+            return
+        self._closed = True
+        previous = api.get_context()
+        self._context.set()
+        try:
+            pending = asyncio.all_tasks(self._loop)
+            for task in pending:
+                task.cancel()
+            # Bound cleanup work; tasks must cooperate with cancellation.
+            for unused in range(3):
+                if not any(not task.done() for task in pending):
+                    break
+                self._pump()
+            for task in pending:
+                if not task.done():
+                    self._loop.call_exception_handler({
+                        'message': 'Script task ignored cancellation during unload',
+                        'task': task,
+                    })
+        finally:
+            self._loop.close()
+            previous.set()
+
+    def _tick(self, userdata):
+        if self._closed:
+            return False
+        previous = api.get_context()
+        if not self._context.set():
+            self._close()
+            self._plugin.remove_hook(self._unload_hook)
+            return False
+        try:
+            self._pump()
+            if not self._task.done():
+                return True
+            try:
+                self._task.result()
+            except asyncio.CancelledError:
+                pass
+            except Exception:
+                traceback.print_exc()
+            self._close()
+            self._plugin.remove_hook(self._unload_hook)
+            return False
+        except Exception:
+            traceback.print_exc()
+            self._close()
+            self._plugin.remove_hook(self._unload_hook)
+            return False
+        finally:
+            previous.set()
+
+
+def run_async(coroutine, context=None, interval=10):
+    """Schedule a coroutine in a captured context; return an AsyncTask handle."""
+    interval = operator.index(interval)
+    if interval < 1 or interval > 2147483647:
+        raise ValueError('interval must be a positive C int in milliseconds')
+    if not asyncio.iscoroutine(coroutine):
+        raise TypeError('run_async expects a coroutine object')
+    plugin = api.__get_current_plugin()
+    return AsyncTask(coroutine, plugin, context or api.get_context(), interval)