Firecrown likelihood for the tomographic ξ± layout + CAMB↔CCL cross-check #248

Row 5 of the PRD #241 §7 table — the theory engine the blinding box (PR 6) will call.

Scope:

• minimal firecrown likelihood for our tomographic ξ± layout: consumes the analysis SACC file; build_likelihood() + compute_theory_vector() is the whole surface Smokescreen uses; N-bin generic
• fiducial cosmology + model configuration (halofit version, IA model, neutrino treatment) as one named, clearly-configurable default set — the PRD §5 open question changes config values, not code
• CAMB↔CCL cross-check test: same cosmology, same n(z), ξ± compared on our θ grid against a stated tolerance, σ8-vs-A_s amplitude convention settled inside the test

Not in scope: Smokescreen wiring, secret handling, blinded-COSEBIs derivation (PR 6).

— Claude (Fable) on behalf of Cail.