Function register_session 

pub fn register_session(
    roost_url: &str,
    session_id: &str,
    capsule_name: &str,
    capsule_version: &str,
    grant: Option<&SpawnApproval>,
) -> Result<SpawnCredential, RuntimeError>

Announce a running session to the daemon and take the credential it mints.

grant is the approval a delegated child was launched with; it is presented in SPAWN_APPROVAL_HEADER and redeemed exactly once by the daemon against the artifact it resolves for capsule_name/capsule_version. None is a top-level launch, which the daemon admits only for a name in its own --spawn-allow.