Function append_json_record_once 

pub fn append_json_record_once(
    root: &Path,
    path: &Path,
    key: &str,
    record: &Value,
    mode: u32,
) -> Result<(), Error>

Writes a local JSONL outbox delivery once across retries and processes. A separate OS-locked companion survives atomic destination replacement. Both reading and replacement are bounded to 64 MiB; incomplete records fail closed instead of losing existing output or duplicating a delivery.