Merge the tomography branch into develop

[cailmdaley]

develop (D) and feature/sp_validation-extend-to-tomography (T) have diverged since #249.

• D has the SACC writers (cosmo_val + Snakemake migration to the SACC writers (PRD #241 row 4) #251) and a run of fixes.
• T has the redesigned tomographic API for pseudo-Cℓ, ξ± and ρ/τ.

New D fixes keep landing on the old API (#366, #372). This epic brings the two lines back together.

Merge order and review gates

Step	What	Lands on	Merges once
1	Pause D PRs that touch the analysis code (cosmo_val/{pseudo_cl,real_space,psf_systematics,pure_eb}.py, rho_tau.py, pseudo_cl.py); #366 and #367 wait	—	—
2a	#253 (blinding)	D	Sacha approves. It is independent of T and already approved by Lisa.
2b	#307 (tomographic leakage)	T	Cail approves (it is Sacha's PR).
3	Merge D into T, porting #366 and #367 (#374)	T	Sacha approves, and Lisa approves the real-space/ξ± part.
4	Merge T into D, then retire T	D	Cail approves. It contains only what steps 2b and 3 already reviewed.
5	Rebase #306 and #256 onto D	D	Normal review. #306 extends D's xi_to_sacc to tomographic bins rather than adding a second writer.

Steps 2a and 2b can go in parallel. Step 3 starts once both have landed.

#372 is closed: T already has that fix.

— Claude on behalf of Cail