Spreadsheet-grade loan amortization for 8 loans incl. rate reset and extra payments

# Task: loan amortization (spreadsheet-grade precision) `loans.csv` lists 8 loans: principal, annual_rate_pct (nominal, compounded monthly), years, extra_monthly (extra principal paid each month from `extra_from_month` on, 1-based), optional `rate_reset_month` + `new_rate_pct`. Rules (follow exactly, use decimal arithmetic, round HALF-UP to cents at each step): 1. Scheduled payment = P·i/(1−(1+i)^−n) with i = rate/12/100, n = years·12, rounded to cents. If rate is 0, payment = P/n rounded. 2. Each month: interest = round(balance·i); principal part = payment − interest; plus the extra payment if active. 3. If principal part + extra ≥ remaining balance, the final payment = interest + remaining balance (no extra that month) and the loan ends. 4. At `rate_reset_month` (before computing that month's interest), switch to the new rate and recompute the payment on the current balance over the ORIGINAL remaining term (n − reset_month + 1 months). Write `results.json`: {"L1": {"monthly_payment": "1234.56" (the initial scheduled payment, string), "total_interest": "…" (string, 2dp), "payoff_month": int}, ...} and `L3_schedule.csv` with header `month,payment,interest,principal,balance` (payment includes extra; principal includes extra; strings with 2dp). No network.