Function serve 

pub async fn serve(
    registry: Registry,
    roster: Roster,
    root: Root,
    define: Option<DefineHandler>,
    listener: UnixListener,
) -> Result<()>

Bind path and serve the root session plus roster until the listener errors.

root is served first and labelled by its own SessionId (the agent’s session id — not the registry’s agent:orchestrator alias). Every session named by roster is served too, now and as it grows. A client addresses a session by frame.session; a server whose current served set is exactly one session also accepts any id, preserving the single-session flow (the --socket client’s placeholder "remote" still reaches it). define, when installed, answers Request::Define — the composition root’s worker factory.

registry is consulted (read-only) for each served session’s model, so the roster this server pushes names models the server actually knows.