Function classify_loginctl 

pub(super) fn classify_loginctl(
    session: &str,
    success: bool,
    code: Option<i32>,
    stdout: &str,
    stderr: &str,
) -> Result<ProbeOutcome, String>

Classify one loginctl invocation. Ok(outcome) ends the probe; Err(detail) means “this candidate failed, try the next” and carries the failure detail — including loginctl’s stderr, which the old code dropped, so a bug report now pins the exact cause (#191). Pure over the raw output pieces so the success classification and stderr capture are testable without spawning loginctl.