Function explain_batch 

pub fn explain_batch(
    input: impl BufRead,
    output: impl Write,
    engine: Option<&dyn DecisionEngine>,
    settings: impl Into<GateSettings>,
) -> Result<()>

hikmah gate-explain --batch: one JSON object per input line ({"id": ..., "last_assistant_message": ...}), one verdict per output line, input order kept.