Clean up the boundaries between shear_psf_leakage, cs_util & sp_validation

[cailmdaley]

Umbrella for getting a cleaner split across the three UNIONS weak-lensing repos: shared concerns live in the shared library (cs_util), analysis/validation logic stays in the analysis repos, and dependency arrows point analysis → library, never the reverse.

• cs_util — shared library of tested primitives (size, shape, cosmology, plotting)
• sp_validation — catalogue validation + cosmology inference
• shear_psf_leakage — PSF-leakage calculation

The same concerns keep getting re-implemented in the wrong layer. Sub-issues track each instance:

• Move cosmology into cs_util.cosmo — Consolidate cosmology into cs_util (single home for the collaboration's cosmology primitives) #75 (two PRs: feat(cosmo): consolidate sp_validation cosmology theory-curve machinery into cs_util.cosmo #76 + refactor(cosmology): move cosmology into cs_util.cosmo; delete local module sp_validation#223).
• Consolidate plot styles into cs_util — shear_psf_leakage/plot_style.py and sp_validation/plots.py both set mpl.rcParams at import; cs_util/plots.py has utilities but no shared style home.
• More as we find them.

— Claude on behalf of Cail