Add Rural Fuel Duty Relief Scheme

[vahid-ahmadi]

Summary

• Implements the Rural Fuel Duty Relief Scheme (closes Rural Fuel Duty Relief Scheme #676).
• Adds a new in_rural_fuel_duty_relief_area household-level input (bool, default False) so that households in eligible postcodes — Inner/Outer Hebrides, Northern Isles, Islands in the Clyde, Isles of Scilly, and listed parts of Cumbria, Devon and Northumberland — can be flagged.
• When flagged, fuel_duty is computed at petrol_and_diesel - rural_fuel_duty_relief (5p/L from 2012-03-01) instead of the headline rate. Applies to both petrol and diesel litres.
• New parameter gov.hmrc.fuel_duty.rural_fuel_duty_relief with HMRC Notice 2001 + SI 2011/2935 references.

Scope notes

• This is the calculator-side change only. The flag defaults to False so household microsim aggregates are unchanged unless data assigns the flag (no FRS postcode info available today). Useful immediately for the household calculator and for reforms that toggle eligibility.
• No change to incidence assumptions — only the effective per-litre rate.

Test plan

• Added 3 YAML cases (policyengine_uk/tests/policy/baseline/gov/hmrc/fuel_duty/rural_fuel_duty_relief.yaml):
  • Non-relief household → full rate.
  • In-relief household, petrol only → 5p/L reduction.
  • In-relief household, mixed petrol + diesel → 5p/L reduction on combined litres.
• Full local policy suite: policyengine-core test policyengine_uk/tests/policy -c policyengine_uk → 1004 passed.
• ruff format + ruff check on touched files → clean.

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