Use road fuel volumes for fuel spending uprating

[MaxGhenis]

Summary

• add an OBR/HMRC road-fuel-volume growth index to PE-UK economic assumptions
• use that index for petrol_spending and diesel_spending instead of CPI
• document and test the runtime fuel spending uprating path

Tests

• uv run ruff check policyengine_uk/tests/test_road_fuel_volume_uprating.py policyengine_uk/variables/input/consumption/petrol_spending.py policyengine_uk/variables/input/consumption/diesel_spending.py
• uv run pytest policyengine_uk/tests/test_road_fuel_volume_uprating.py policyengine_uk/tests/code_health/test_input_variable_definitions.py -q
• git diff --check