Rebuild two years of a repayment mortgage statement with a rate change, overpayments and a bounced payment

# Rebuild a mortgage statement Inputs: `loan.json` (opening balance on 2024-11-01, starting annual rate, contractual monthly payment), `rate-notices.csv` (rate changes) and `payments.csv` (everything that hit the mortgage account, including returned direct debits). Rebuild the 24 monthly statements from November 2024 to October 2026. - Each month: interest = opening balance x annual rate / 12, rounded to the penny half-up, and added to the balance. Then every payment for that month that was not returned comes off the balance. - A rate notice applies from the month in its `effective_month` (that month's interest already uses the new rate). - A payment whose `status` is `returned` never reached the lender; ignore it. Overpayments are payments like any other. - Keep the balance in exact pence; there is no other rounding. Write `out/statement.json`: `{"closing_balance": <balance after October 2026>, "total_interest": <sum of the 24 monthly interest charges>, "interest_by_year": {"2024": <...>, "2025": <...>, "2026": <...>}, "total_paid": <sum of payments counted>, "interest_after_rate_change": <interest charged from the first rate-change month onwards>}`. ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.