Function write_solutions_json 

pub fn write_solutions_json<W: Write>(
    w: &mut W,
    m: &Machine,
    exhausted: bool,
    output: Option<&str>,
) -> Result<()>

v1 success object: {"count":N,"exhausted":B,"solutions":[…]}, keys in serde_json sorted order. output, when Some, inserts an "output" field (sorts between exhausted and solutions) carrying captured write/1 bytes — the reactor uses it (no stdout in an isolate, D4); the CLI passes None because its output already streamed to stdout, keeping native bytes byte-identical to v1.