Seed EntitledTo-style integration tests (#778)

[vahid-ahmadi]

Summary

• First batch of worked benefit scenarios for Integration tests from EntitledTo #778, which asks for ~20 EntitledTo-aligned integration tests.
• New file at policyengine_uk/tests/policy/integration/entitledto_scenarios.yaml with three common household shapes:
  1. Single 30-year-old working part-time on low pay, private renter in London — exercises UC, no income tax / NI, child benefit zero.
  2. Couple with two children, one earner at £22k, social housing in North West — exercises UC, child benefit, partial income tax + NI (asserted via household_tax).
  3. Pensioner couple with low state pension (£14k combined) — sits below the Pension Credit guarantee level; exercises Pension Credit and Winter Fuel Allowance.
• Values are pinned to PolicyEngine's current output. The file is labelled as a regression-grade baseline rather than an authoritative EntitledTo cross-validation — future work is to manually re-run the matching cases through EntitledTo and tighten any rows where the two disagree, ratcheting up to the ~20-scenario coverage in Integration tests from EntitledTo #778.
• Person-level variables (income_tax, national_insurance) can't be asserted with a single scalar across a multi-person benunit, so scenario 2 uses household_tax (the aggregate that already includes both).

Test plan

• policyengine-core test policyengine_uk/tests/policy/integration/entitledto_scenarios.yaml -c policyengine_uk (3 passed).

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