Smokescreen blinding wiring (fork protocol, three theory backends, custody)

Wire Smokescreen blinding into sp_validation on the fork's theory-backend protocol: three sp_validation theory_fn backends driven by one deterministic per-version draw, per-part-at-birth blinding through a three-verb custody surface, and a hash commitment that stays publicly verifiable while the seed and true vectors sit encrypted at rest.

Desired end state

Custody — per part, at birth. Three verbs plus one assertion:

• blind-init (once per catalogue version): OS-entropy seed → repo-committable commitment.json carrying sha256(seed) plus a config digest binding envelope and fiducial → seed encrypted into a bundle (smokescreen.encryption, no firecrown dependency). The plaintext seed is never written to disk.
• blind-part (per blindable intermediate — coarse ξ±, fine ξ±, pseudo-Cℓ — the moment it is computed): read the encrypted seed, conceal the part, stamp concealed=True and blind_commitment, strip seed_smokescreen, escrow the true vector into a per-part bundle beside the blinded part, delete the plaintext. Each part's escrow is self-contained: unblinding one part needs only its bundle plus the seed bundle.
• unblind: verifies both sha256(seed) and the config digest against the commitment before subtracting — fails closed on either mismatch.
• Terminal assembly asserts blind_commitment is identical across parts, failing closed otherwise.

Derived statistics. COSEBIs and pure-E/B are born blinded — re-derived through the pipeline estimators from the blinded fine ξ±, never shifted directly. Covariances never change. The pure-EB seam follows the pipeline's edge-based bounds.

On the fork protocol.

• The amplitude-shift envelope is expressed through the fork's fixed CCL-native draw (order-independent, local default_rng), calibrated on the sp_validation side to an equivalent S8 amplitude; the same seed yields the same hidden cosmology for every part, which is what makes the parts mutually consistent.
• Three theory backends — coarse ξ±, fine-grid ξ±, and pseudo-Cℓ (W @ ΔCℓ_EE) — each a theory_fn callable supplied by this issue, overriding the fork's built-in default CCL backend (which does not know our layout or IA config). Our backends are plain CCL callables that compute exactly our fiducial.
• Cross-backend consistency: the same hidden cosmology through two theory callables agrees to machine precision (~1e-10).

CAMB↔CCL cross-check (folded-in test). A theory-consistency test — same cosmology, same n(z), ξ± on our θ grid within tolerance. It must settle the σ8-for-CCL vs A_s-for-CAMB amplitude convention (σ8-matching is load-bearing: nominal A_s leaves σ8 ~3% off and blows the comparison to ~9–10%), and halofit_version strings must match on both stacks (mead2020 vs mead2020_feedback differ several percent at k≳1). Observed floor on the single-bin synthetic fixture: ξ+ 0.21% / ξ− 0.10% against 0.5%/1.0% tolerances.

Out of scope: Snakemake rule authoring — which rule invokes blind-part, where blind-init fires — belongs to the migration row (#247). This issue delivers the CLIs, backends, and the assembly-time assertion.

Acceptance

• B-mode estimators are unchanged under blinding on mocks, to the estimator's numerical floor.
• End-to-end on fixture parts: blind-init → blind-part on each of the three real intermediates → terminal assembly (hash assertion passes) → unblind restores each part bit-for-bit; no plaintext seed or true part survives on disk at any point.
• The cross-backend and CAMB↔CCL consistency tests pass at the stated tolerances.

— Fable on behalf of Cail.

Start by locating the sp_validation theory_fn protocol, the three intermediate representations (coarse ξ±, fine-grid ξ±, and pseudo-Cℓ), and the existing CLI and fixture-test entry points. Trace how the three backends and custody artifacts should connect, then use the stated end-to-end, cross-backend, CAMB↔CCL, and B-mode acceptance tests to define completion.