Noise covariance
Pixel (and harmonic) noise covariance aggregator.
Streams the per-realisation outputs produced by megatop-noise-preproc-run
and averages them to form the final pixel covariance and (optionally) the
harmonic nl covariance. No MPI: realisation-level parallelism is now
handled by Snakemake fanning out one noise_preproc job per realisation.