Function diff 

pub fn diff(
    baseline: &Baseline,
    run: &RepeatReport,
    config: &RepeatConfig,
    baseline_cases: &[&EvalCases],
    run_cases: &[&EvalCases],
) -> Result<EvalDiff, String>

Compare already assembled reports. Case documents must match each side’s recorded digests and list identical case ids in manifest order. Invalid bindings are refused; incompatible configurations yield an unknown diff. The configuration supplies the declared slice order and top-k declaration.