Impute student loan balances from WAS

[MaxGhenis]

Summary

• derive student_loan_balance in the WAS wealth-prep table as total loans minus loans excluding SLC
• include student_loan_balance in the wealth imputation outputs
• retrain the cached wealth model automatically when the stored artifact is missing that new output column

WAS diagnostics

• positive-balance households in Round 7: 701 unweighted
• weighted households with SLC debt: 1.664m
• weighted total SLC debt: £33.4bn
• weighted mean balance among households with debt: £20.1k
• median positive balance: £14.0k
• raw subtraction produced 0 negative rows, so clipping was not needed

Testing

• uvx ruff check policyengine_uk_data/datasets/imputations/wealth.py policyengine_uk_data/tests/test_student_loan_balance.py
• uv run --python 3.13 pytest -q policyengine_uk_data/tests/test_student_loan_balance.py

Closes #238.