Check for consistency between UC and legacy benefit `claims_*` variables

[MaxGhenis]

To avoid issues like #200

[MaxGhenis]

Is this still an issue that can be addressed in openfisca-uk? Seems like it's good on the PolicyEngine front.

[vahid-ahmadi]

Largely addressed in current architecture. Audit:

1. active.yaml flags shut legacy schemes off at managed-migration completion:

• tax_credits/active.yaml → false from 2025-04-06 (PR Stop paying Tax Credits after DWP managed migration completed #1619).
• income_support/active.yaml, JSA/income/active.yaml → false from 2026-04-01 (PR Zero out Income Support and income-based JSA after managed migration #1631).

After managed migration completes, no legacy benefit can be awarded alongside UC.

2. Pre-migration consistency uses reported anchors:

• claims_legacy_benefits — true if any of CTC/WTC/HB/ESA-income/IS/JSA-income is reported.
• claims_all_entitled_benefits — true only when no benefits are reported.
• The legacy would_claim_* formulas combine these (e.g. reported_IS | claims_all_entitled_benefits), and jsa_income / esa_income further constrain to the reported amount, so an unreported family can't synthesise legacy awards on top of UC.

3. Code-level mutual-exclusion check:

• housing_benefit_eligible explicitly includes & ~claiming_uc.

This was the only place the consistency check sat in code rather than in data anchoring, and it wasn't pinned by a test. PR #1638 adds yaml regression tests so a future refactor can't silently break the HB ↔ UC exclusion.

Closing once #1638 lands.