TCP listener for one PeerNet instance. Accepts inbound connections and spawns one PeerNet.Connection per accepted socket.

The acceptor is intentionally tiny — all the state lives on the per-connection process. The acceptor's only job is to call :gen_tcp.accept/1 in a loop and hand each accepted socket off to a fresh connection process.

Bind on port 0 to let the OS pick an ephemeral port; read it back with port/1 afterwards (used in tests and in places where the listener's address needs to be advertised externally).