cosmo: absorb generic theory machinery from sp_validation blinding (CCL ξ± path, CAMB cross-path, σ8/A_s rescale)

cs_util.cosmo is the cosmology library for the UNIONS/CosmoStat stack; survey-specific configuration and implementations live in the consuming packages (division settled with Cail: cs_util = generic cosmology machinery; sp_validation etc. = configuration + survey-specific implementations).

The sp_validation blinding work (CosmoStat/sp_validation#252) is accumulating generic theory machinery that belongs here once stable:

• a CCL-native ξ± path with configurable nonlinear model (HMCode2020 via CAMB, halofit_version token, logT_AGN), two-tracer cross-pairs, NLA IA
• an independent CAMB P(k) → ccl.Pk2D → CCL projection path (stack cross-validation)
• the closed-form σ8↔A_s amplitude rescale (A_s = A_s_seed · (σ8_target/σ8_seed)²)

These currently live in sp_validation's blinding namespace with a docstring marking them cs_util-bound. Graduate them here once the blinding PR lands and the interfaces have settled; consumers beyond blinding: image-simulation imprints, inference theory checks.

— Fable on behalf of Cail.

Start by reviewing the generic cosmology machinery described in sp_validation's blinding namespace and the dependency on sp_validation#252. Compare the CCL-native ξ± path, CAMB cross-validation path, and σ8↔A_s rescale with the cs_util boundary. Done means the stable generic machinery is moved into cs_util while survey-specific configuration remains in sp_validation.