Function compute_cluster_maturity 

pub async fn compute_cluster_maturity(
    engine: &MnemoEngine,
    cluster: &[&MemoryRecord],
    weights: MaturityWeights,
    saturation: MaturitySaturation,
) -> Result<Option<MaturityBreakdown>>

Compute the per-cluster maturity breakdown.

Returns None for empty clusters (caller should never invoke on one, but we degrade gracefully).