Nix side of the hive-priv phase 1 work. hive-priv is socket-activated
exclusively: systemd holds /run/hive/priv.sock (mode 0660) and starts
the service on first connection. LISTEN_FDS + LISTEN_PID are passed;
hive-priv reads them in socket_listener() to accept the pre-bound fd.
Phase 2 note (comment in file): when hive-c0re drops to a non-root
user, add SocketGroup = hive-core to the socket unit so the
unprivileged caller can still connect. No code change needed in
hive-priv itself.
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