Add Parents' Learning Allowance model

[MaxGhenis]

Summary

• add a first-pass Student Finance England Parents' Learning Allowance model
• add published 2025/26 and 2026/27 maximum-amount and household-income-cutoff parameters
• wire the new grant into spending, household-benefit, HBAI-benefit, and program coverage aggregates

Model boundary

This is a partial model.

The public GOV.UK pages publish:

• the household-income cutoff
• the maximum annual amount
• the fact that awards range between £50 and the maximum

But they do not publish a simple public award curve. Rather than inventing a taper, this branch pays the published maximum amount to eligible students and makes that proxy explicit in the variable documentation and program notes.

The default course-eligibility proxy uses higher-education evidence, with an explicit override available for 120-credit and ITT edge cases.

Testing

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