Consolidate epoll, kqueue, and select into shared reactor layer

Introduce a shared reactor/ directory with CRTP templates that capture
the common structure of all three POSIX backends: operation state,
descriptor state, socket/acceptor impls, service ownership, and the
full scheduler threading model.

New shared headers:
- reactor_op.hpp: base op with cancellation, stop_token, perform_io
- reactor_descriptor_state.hpp: per-fd state with deferred I/O
- reactor_socket.hpp: connect, read_some, write_some, cancel, close
- reactor_acceptor.hpp: bind, listen, accept, cancel, close
- reactor_service_state.hpp: service ownership (mutex, list, map)
- reactor_scheduler.hpp: signal state machine, inline completion
  budget, work counting, run/poll event loop, cleanup guards

Backend files become thin wrappers providing only platform-specific
hooks: write policy (sendmsg flags), accept policy (accept4 vs
accept+fcntl), and the reactor poll function (epoll_wait, kevent,
select). All shared logic — speculative I/O, deferred I/O dispatch,
cancellation, close, thread coordination — lives in the templates.

Correctness fixes applied during normalization:
- EINTR retry on kqueue read path
- Release ordering on add_ready_events/is_enqueued_ (ARM correctness)
- Exception-safe task_cleanup with private queue drain
- stopped_ as plain bool (always under mutex)

Author: sgerbino

include/boost/corosio/native/detail/epoll/epoll_acceptor.hpp

@@ -14,23 +14,22 @@
 
 #if BOOST_COROSIO_HAS_EPOLL
 
-#include <boost/corosio/tcp_acceptor.hpp>
-#include <boost/capy/ex/executor_ref.hpp>
-#include <boost/corosio/detail/intrusive.hpp>
-
+#include <boost/corosio/native/detail/reactor/reactor_acceptor.hpp>
 #include <boost/corosio/native/detail/epoll/epoll_op.hpp>
-
-#include <memory>
+#include <boost/capy/ex/executor_ref.hpp>
 
 namespace boost::corosio::detail {
 
 class epoll_acceptor_service;
 
 /// Acceptor implementation for epoll backend.
 class epoll_acceptor final
-    : public tcp_acceptor::implementation
-    , public std::enable_shared_from_this<epoll_acceptor>
-    , public intrusive_list<epoll_acceptor>::node
+    : public reactor_acceptor<
+        epoll_acceptor,
+        epoll_acceptor_service,
+        epoll_op,
+        epoll_accept_op,
+        descriptor_state>
 {
     friend class epoll_acceptor_service;
 
@@ -44,47 +43,8 @@ class epoll_acceptor final
         std::error_code*,
         io_object::implementation**) override;
 
-    int native_handle() const noexcept
-    {
-        return fd_;
-    }
-    endpoint local_endpoint() const noexcept override
-    {
-        return local_endpoint_;
-    }
-    bool is_open() const noexcept override
-    {
-        return fd_ >= 0;
-    }
     void cancel() noexcept override;
-
-    std::error_code set_option(
-        int level,
-        int optname,
-        void const* data,
-        std::size_t size) noexcept override;
-    std::error_code
-    get_option(int level, int optname, void* data, std::size_t* size)
-        const noexcept override;
-    void cancel_single_op(epoll_op& op) noexcept;
     void close_socket() noexcept;
-    void set_local_endpoint(endpoint ep) noexcept
-    {
-        local_endpoint_ = ep;
-    }
-
-    epoll_acceptor_service& service() noexcept
-    {
-        return svc_;
-    }
-
-    epoll_accept_op acc_;
-    descriptor_state desc_state_;
-
-private:
-    epoll_acceptor_service& svc_;
-    int fd_ = -1;
-    endpoint local_endpoint_;
 };
 
 } // namespace boost::corosio::detail