tcp_socket missing bind() method

[sgerbino]

Summary

tcp_socket does not expose a bind() method, while udp_socket and tcp_acceptor both do. The underlying infrastructure (reactor_basic_socket::do_bind()) supports it for all socket types — it's just not wired through the TCP path.

Current state

	tcp_socket	udp_socket	tcp_acceptor
open()	Yes	Yes	Yes
bind()	No	Yes	Yes

• udp_service has bind_datagram() virtual — tcp_service has no equivalent
• reactor_basic_socket::do_bind() works for both TCP and UDP
• Native variants (native_tcp_socket, native_udp_socket) inherit non-async ops from the type-erased base, so the gap propagates uniformly

Impact

A TCP client that needs to bind to a specific local address/port before connecting (multi-homed hosts, source-port pinning) cannot do so at any API level.

Proposed fix

1. Add bind_socket() to tcp_service (virtual interface)
2. Implement in all backends (delegate to do_bind())
3. Expose bind(endpoint) on tcp_socket
4. native_tcp_socket inherits it automatically