Add the UK Budget reform contract runner

changelog.d/576.added.md

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+Added the UK Budget reform contract (`policyengine.tax_benefit_models.uk.budget_reform_contract`), version 1.0.0. `run_budget_reform_contract()` checks that dated reforms (an April rate change, a finite change, an extended freeze, an April uprating schedule, a mid-year blended rate, a CPI-driven indexation change, a scenario or policy combining a parameter-setting modifier with dated changes, an unknown path, and each route's documented key readings) give independently derived results through the policyengine-uk and policyengine.py routes, leave the baseline and later unreformed runs unchanged, and survive bounded property sampling. It returns a JSON report for release qualification.