Function background_output_fact 

pub fn background_output_fact(
    name: &str,
    line: &str,
    task_seq: u64,
    event: Option<&str>,
    stream: Option<&str>,
) -> NewFact

One background-task output line as a fact on the session’s one log.

task_seq counts emitted facts within the task and rides the fact KEY (bg:{name}:{task_seq}), which is what makes keys unique per task without coordinating with the log’s own seq. event is Some("truncated" | "exited") for the stream-cap marker and the final disposition line; stream is Some("stderr") for standard-error lines.