Model balance-aware student loan repayments

[MaxGhenis]

Summary

• add a student_loan_balance input variable for person-level outstanding debt
• cap modelled student loan repayments by positive outstanding balance when provided
• make student_loan_repayments default to the modelled repayment so HBAI and other aggregates use the model path

Testing

• uvx ruff check policyengine_uk/variables/gov/hmrc/student_loans/student_loan_balance.py policyengine_uk/variables/gov/hmrc/student_loans/student_loan_repayment.py policyengine_uk/variables/gov/hmrc/student_loan_repayments.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/student_loans/student_loan_repayment.yaml policyengine_uk/tests/policy/baseline/gov/hmrc/student_loans/student_loan_interest_rate.yaml

Closes #644