No cumulative floating-point errors in multi-step transactions

Floating-point errors in a single step are small; across a multi-step chain they compound unpredictably. A refund pipeline that subtracts a fee, applies a discount, and then taxes the result in float arithmetic will produce a different final amount depending on the order of operations — a determinism failure that makes reconciliation impossible. CWE-682 covers incorrect calculation; ISO 25010 functional-correctness requires financial outputs to be exact. Even a one-cent error per transaction becomes a material accounting discrepancy at tens of thousands of monthly transactions.

Refactor all multi-step calculation chains to operate in integer cents from start to finish. Convert to cents once at the entry point and convert back to decimal only at display time.

// src/lib/finance.ts — integer arithmetic throughout
export function calculateRefund(
  originalCents: number,
  feeCents: number,
  discountCents: number
): number {
  // All intermediates remain integers
  const afterFee = originalCents - feeCents;
  const afterDiscount = afterFee - discountCents;
  return Math.max(0, afterDiscount); // never negative
}

Search src/ for any pattern matching balance += or total -= where the variable is initialized as a float and replace with integer cent equivalents.