Add Childcare Grant model

[MaxGhenis]

Summary

• add a first-pass England Childcare Grant model with parameters, eligibility, and amount logic
• wire Childcare Grant into aggregate benefit/spending outputs and program metadata
• add focused policy coverage for caps, income limits, age rules, and incompatibilities

Testing

• uvx ruff check policyengine_uk/variables/gov/dfe/childcare_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/childcare_grant/childcare_grant.yaml