Function save 

pub fn save(
    store: &ArtifactStore,
    benchmark_id: &str,
    target: &str,
    model: &str,
    metrics: BTreeMap<String, f64>,
) -> Result<()>

2026-09-26: Writes the model-keyed baseline, replacing any earlier one. Call only after a run whose numbers should become the bar: a failed or partial leg stored here is compared against by every later run.