Add Adult Dependants' Grant

[MaxGhenis]

Summary

• add a first-pass England Adult Dependants' Grant model
• parameterize the published maximum, household-income cutoff, and non-partner adult income limit
• wire the new grant into program metadata and aggregate benefit/spending totals

Model shape

This is intentionally a partial model.

• eligible students receive the published maximum amount because GOV.UK does not expose a simple public award curve
• co-resident couple students aged 25+ qualify through an automatic partner path
• under-25 spouse/civil-partner cases and non-partner adult dependant cases are exposed as explicit inputs
• the course path defaults to higher-education evidence rather than a narrower England-finance entitlement rule
• Postgraduate Loan overlap and dependant-adult student-finance overlap both make the student ineligible

Validation

• uvx ruff check policyengine_uk/variables/gov/dfe/adult_dependants_grant policyengine_uk/variables/gov/gov_spending.py policyengine_uk/variables/household/income/household_benefits.py policyengine_uk/variables/household/income/hbai_benefits.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/dfe/adult_dependants_grant/adult_dependants_grant.yaml

Baseline aggregate on current enhanced FRS

Using /Users/maxghenis/PolicyEngine/policyengine-uk-data/policyengine_uk_data/storage/enhanced_frs_2023_24.h5 at 2025:

• recipients: 5.20k
• spend: £18.44m
• recipient mean: £3,545

Part of #460.