Banker's rounding (round-half-to-even) for interest calculations

Standard Math.round() always rounds 0.5 upward, introducing a systematic positive bias in interest calculations. Over millions of compounding periods this bias is non-trivial and legally relevant — financial regulators and ISO 25010 functional-correctness standards expect rounding to be demonstrably unbiased. Banker's rounding (round-half-to-even) is the industry standard because it distributes rounding error evenly across the dataset. CWE-682 covers this class of incorrect calculation leading to incorrect financial results.

Replace bare Math.round() calls in financial calculation logic with a banker's rounding implementation or a library that supports ROUND_HALF_EVEN. Centralize this in src/lib/finance.ts.

// src/lib/finance.ts
import Decimal from 'decimal.js';

export function roundToNearestCent(value: number): number {
  return new Decimal(value)
    .toDecimalPlaces(2, Decimal.ROUND_HALF_EVEN)
    .toNumber();
}

Search for all Math.round, .toFixed, and Math.ceil calls in src/ and replace with roundToNearestCent() for any interest, APY, or fee calculation.