Function build_invariant_progress_json 

fn build_invariant_progress_json<M: Serialize>(
    context: InvariantProgressContext<'_>,
    corpus_metrics: &M,
    failure_metrics: &InvariantFailureMetrics,
) -> Value

Builds the machine-readable invariant progress payload emitted during a campaign.

This keeps the existing corpus progress metrics together with cumulative and derived throughput fields so downstream benchmark tooling can consume a single JSON event shape.