Add 16 to 19 Bursary Fund model

[MaxGhenis]

Summary

• add a first-pass England 16 to 19 Bursary Fund model for defined vulnerable groups
• wire the bursary into aggregate benefit and spending outputs plus program metadata
• add focused policy coverage for care, self-supporting UC, disability routes, apprenticeship exclusion, age extension, and England-only scope

Testing

• uvx ruff check policyengine_uk/variables/gov/dfe/bursary_fund_16_to_19 policyengine_uk/variables/gov/gov_spending.py policyengine_uk/variables/household/income/household_benefits.py policyengine_uk/variables/household/income/hbai_benefits.py
• uvx ruff format --check policyengine_uk/variables/gov/dfe/bursary_fund_16_to_19 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/bursary_fund_16_to_19/bursary_fund_16_to_19.yaml