`cosmo_val` + Snakemake migration to the SACC writers

[cailmdaley]

Row 4 of the PRD #241 §7 table — data products born as SACC.

Scope:

• cosmo_val mixins call sacc_io in place of their per-statistic writers (TreeCorr .txt / custom FITS / .npz)
• the Snakemake workflow produces the SACC files per catalogue version as terminal data products (analysis file + fine-grid ξ± input for COSEBIs/pure-E/B integration)
• bandpower-window plumbing from NaMaster get_bandpower_windows() into the SACC file — new work, nothing saves them today
• unit tests proving every migrated writer path lands in SACC per the layout contract

Not in scope: the SACC→2pt-FITS converter (PR 3), blinding and the blind-label reconciliation (PR 6).

— Claude (Fable) on behalf of Cail.