Improve student loan validation notebook

[MaxGhenis]

Summary

Enhances the student loan repayment validation notebook with deeper analysis and documentation of known limitations.

Changes

• Add correlation analysis among those required to pay (weighted correlation: 0.68)
• Document why overall correlation is low (33% of reporters have income below threshold)
• Add deep dive on zero modelled repayments cases
• Document known limitations:
  • No cap at outstanding balance
  • FRS data issues (voluntary payments, intra-year income variation)
• Add calibration status section noting WAS imputation opportunity
• Fix enum handling for StudentLoanPlan string values
• Clarify that FRS student_loans variable is THIS YEAR borrowing, not outstanding balance

Test plan

• Notebook runs without errors
• Analysis is consistent with previous findings

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