Add a systemd.sockets.hive-c0re unit that holds /run/hyperhive/host.sock
before hive-c0re starts. hive-c0re serve() detects LISTEN_FDS via the
listenfd crate and accepts the systemd-handed fd instead of calling bind().
Falls back to the existing bind path when LISTEN_FDS is absent so direct
invocation and CI are unaffected.
Benefits: hivectl can connect the moment the socket unit activates (no
racy window), and a hive-c0re restart never drops the socket inode.
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