[pa-devbot]

Abstract class only - no backends yet (Qt/POSIX/WinSocket to follow).

Mirrors the shape of AbstractClientSocket:

• One connection at a time. Implementations use one worker thread for the accept loop and one thread-pool thread per active connection; a second incoming connection while one is active is rejected (on_connect_rejected).
• State machine: NOT_RUNNING -> LISTENING -> CONNECTED -> LISTENING, with DESTRUCTING while close() is in flight.
• Listener: on_thread_start, on_listen_finished(error_message) (the result of start(), like on_connect_finished for the client), on_connect, on_connect_rejected, on_receive_data, on_disconnect(error_message) (same semantics as the client's - not fired by close()/close_connection()).
• API: start(address, port) (async, result via listener), close_connection() (drop the client, keep listening), close() (full teardown, idempotent), send() (to the active connection, 0 if none).

Design notes:

• The accept thread hands the connection off to the pool with plain dispatch (not dispatch_now_blocking) - an earlier task blocking on a later task can deadlock the pool (see the note in ThreadPool.h).
• Callbacks run on the worker threads, so close()/close_connection() must not be called from within a listener callback (same contract as ClientSocket).
• Extensions deliberately left out of v1: per-connection ids (for concurrent connections later), the client's address in on_connect, and get_port() for bind-port-0.