UK dated reforms skip fiscal-year processing in Simulation(policy=...) and calculate_household, and dict policies cannot carry an end date #572

Summary

UK dated reforms through policyengine.py give a different answer from the same reform written as a policyengine-uk statutory change.

1. Simulation(policy=...) builds an unreformed Microsimulation, then applies ParameterValues through simulation_modifier_from_parameter_values. That edits the already-annualised parameters (tax_benefit_models/uk/model.py, run). So a change from 6 April misses that fiscal year, and derived parameters stay on the baseline: indexed thresholds, aligned NICs thresholds, and anything uprated from a changed forecast. Reforms also arrive after policyengine-uk has computed its UC rebalancing inputs and parameter-gated structural reforms, so reforms of those parameters do nothing.
2. pe.uk.calculate_household(reform=...) passes dated keys straight to policyengine-uk's dict route, which reads a lone "YYYY-MM-DD" key as a one-day override. So {"gov.hmrc.income_tax.rates.uk[0].rate": {"2027-04-06": 0.21}} changes nothing. Simulation(policy=...) reads the same key as an effective date.
3. The dict compiler (common/reform.py, _reform_dict_to_parameter_values) parses keys strictly as %Y-%m-%d, so a finite change ("2027-04-06.2028-04-05") cannot be written as a dict. The sim API therefore drops stop dates before calling the wrapper (PolicyEngine/policyengine-sim-api, _normalise_period_key).
4. tests/test_household_impact.py only asserts that a UK child benefit reform returns floats, so it passes when the reform has no effect.

Fix

• Pass UK policies to policyengine-uk as an explicit-interval reform dict. policyengine-uk (Apply dated reforms on the statutory timeline on every route policyengine-uk#2305) then writes them onto the statutory timeline and processes them as current law.
• Give the household route the same effective-date reading as Simulation(policy=...).
• Accept "start.stop" interval keys in dict policies, and reject bare years with an explanation.

Part of the UK Autumn Budget 2026 CI plan (workstream W1).