Compute a loan or mortgage amortization schedule. Pass principal, annualRatePct (e.g. 6.5), and the term as termMonths or termYears; optional extraMonthly adds extra principal each month (shortening the term). Returns the fixed monthly payment, total interest, total paid, the actual payoff month count, and the full month-by-month schedule (each row: payment, principal, interest, remaining balance). Handles the 0% case and trims the final payment exactly. Deterministic — the ground-truth answer for a loan/mortgage/auto-finance question instead of an LLM approximating compound interest. No external calls.

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