Migrate cosmo_val + Snakemake to the SACC writers #261

Make SACC the sole native output format of the cosmology-validation pipeline. Every two-point product (ξ± coarse and fine, pseudo-Cℓ, COSEBIs, pure-E/B, ρ/τ, n(z)) is written through the sacc_io builders into single-statistic SACC part files, and the Snakemake workflow assembles those parts into the two terminal SACC files per catalogue version. No bespoke writer remains on the data-vector path.

Desired end state

• Each cosmo_val statistic's write step is replaced by a call to the matching sacc_io builder, producing a part file that holds exactly one statistic plus the self-describing n(z) tracers it references. A producer writes a part with a single save(s, path) call; it never assembles a terminal file and never emits a bespoke format alongside.
• A dedicated Snakemake assembly rule reads the parts, concatenates them in canonical order, attaches the assembled covariance, and writes the two terminal files: {version}.sacc (analysis vector, FullCovariance) and {version}_xi_fine.sacc (fine ξ± grid, DiagonalCovariance from TreeCorr variances).
• The bespoke .txt/custom-FITS/.npz writers on the data-vector path are deleted; nothing downstream reads them.

sacc_io is a fixed dependency and the single source of truth for every builder signature. Metadata keys required on every file: catalogue_version, sp_validation_version, created, npatch.

Acceptance

The workflow produces both terminal files per version from parts; the deleted bespoke writers have no remaining readers; the assembled files match the sacc_io layout contract.

— Opus on behalf of Cail.