feat: add CancelIoEx support for accept operations on Windows

- Add virtual do_cancel() hook to overlapped_op base class
- Implement accept_op::do_cancel() calling CancelIoEx on listen socket
- Wire canceller to invoke do_cancel() after request_cancel()

Author: vinniefalco

src/detail/win_iocp_sockets.cpp

@@ -75,6 +75,18 @@ operator()()
     d(h).resume();
 }
 
+void
+accept_op::
+do_cancel() noexcept
+{
+    if (listen_socket != INVALID_SOCKET)
+    {
+        ::CancelIoEx(
+            reinterpret_cast<HANDLE>(listen_socket),
+            this);
+    }
+}
+
 win_socket_impl::
 win_socket_impl(win_iocp_sockets& svc) noexcept
     : svc_(svc)

src/detail/win_iocp_sockets.hpp

@@ -73,6 +73,9 @@ struct accept_op : overlapped_op
 
     /** Resume the coroutine after accept completes. */
     void operator()() override;
+
+    /** Cancel the pending accept via CancelIoEx. */
+    void do_cancel() noexcept override;
 };
 
 //------------------------------------------------------------------------------

src/detail/win_overlapped_op.hpp

@@ -36,7 +36,11 @@ struct overlapped_op
     struct canceller
     {
         overlapped_op* op;
-        void operator()() const noexcept { op->request_cancel(); }
+        void operator()() const noexcept
+        {
+            op->request_cancel();
+            op->do_cancel();
+        }
     };
 
     capy::any_coro h;
@@ -108,6 +112,11 @@ struct overlapped_op
         cancelled.store(true, std::memory_order_release);
     }
 
+    /** Hook for derived classes to perform actual I/O cancellation. */
+    virtual void do_cancel() noexcept
+    {
+    }
+
     void start(std::stop_token token)
     {
         cancelled.store(false, std::memory_order_release);