BandwidthSocket

The callback surface a bandwidth consumer exposes to a BandwidthManager: pure-Kotlin port of libtorrent's bandwidth_socket (aux_/bandwidth_socket.hpp).

In libtorrent the only implementer is peer_connection. The manager is a pure scheduling algorithm and never touches sockets directly; it just calls back through this interface to hand assigned quota to the consumer and to ask whether the consumer is shutting down. Keeping it abstract lets the limiter be tested in isolation and ported without any I/O.