Update PolicyEngine UK to 2.88.1

[policyengine]

Summary

Update PolicyEngine UK from 2.78.0 to 2.88.1.

What changed (2.78.0 -> 2.88.1)

Added

• Support Python 3.9 and 3.10 (in addition to 3.11–3.14). On Python 3.9/3.10, pip resolves policyengine-core to a version that has been relaxed to support older Python (3.24.0+); on 3.11+ behavior is unchanged.
• • Add a microsimulation smoke test suite that runs against the unpinned latest enhanced FRS dataset and asserts plausibility bounds for UK population, UC aggregate, is_parent population, core benefit totals, and extended childcare eligibility. Catches silent model/data skew at the point the dataset is republished, not after a release.
• Add a first-pass Disabled Students' Allowance model for England higher-education students.
• Add a first-pass Travel Grant model.
• Add a first-pass Adult Dependants' Grant model.
• Add a first-pass Parents' Learning Allowance model.
• Add a first-pass 16 to 19 Bursary Fund model for vulnerable groups.
• • Add a first-pass Childcare Grant model for England full-time undergraduates.
• • Add an approximate England full-time maintenance loan model, with explicit override inputs for living arrangement and assessed household income.
• • Add an approximate England full-time maintenance loan model, with explicit override inputs for living arrangement and assessed household income.
• Add named economic-assumption parameters for the local-authority ONS income target uprating factors used by policyengine-uk-data.

Changed

• • Update .github/CONTRIBUTING.md to document the towncrier changelog.d/ workflow. The old changelog_entry.yaml + make changelog flow was deprecated some time ago; the CONTRIBUTING guide still instructed new contributors to use it, causing CI round-trips on PRs that created a changelog_entry.yaml no fragment step was looking for.
• Expose build metadata helpers for UK data artifacts, including a stable data-build fingerprint and build provenance metadata.

Fixed

• • Fix state_pension_type incorrectly classifying every pensioner as receiving the pre-2016 basic State Pension. The formula used values_list[0] to find when the New State Pension activated, but policyengine-core auto-extrapolates the parameter into the far future, so [0] was returning a 2040s entry instead of the 2016 activation date. Walks the list oldest-first to find the real activation instant, so post-2016 retirees are now correctly classified as NEW. Raises the modelled 2025 state pension aggregate from about £116bn to about £127bn.
• • Fix Working Tax Credit and Child Tax Credit continuing to pay out from the 2025-26 tax year onward. Working Tax Credit and Child Tax Credit ended on 5 April 2025 (HMRC/DWP). Adds a gov.dwp.tax_credits.active parameter that flips to false on 2025-04-06 and gates tax_credits on it. Removes about £1.9bn of phantom Tax Credit spending per year from 2025-26 onward while preserving the legitimate 2024-25 baseline.
• Fix maintenance loan proxy logic to require explicit higher-education evidence and improve sponsor-income assessment.
• Use modelled student loan repayments in aggregates and cap them by outstanding balance when available.
• Fixed corporate_land_value to allocate aggregate corporate land using the current weighted distribution of corporate_wealth, and refreshed the aggregate land parameters to the 2024 ONS land totals used by policyengine-uk-data.
• Corrected Universal Credit rebalancing so existing health-element claimants keep their combined standard allowance and health element award CPI-protected.

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