Interest/APY rounded consistently

Two calculation sites that both compute interest but use different rounding functions — one Math.round(), one Math.floor() — will produce different balances for identical inputs. The discrepancy shows up as a mismatch between the displayed APY and the actual accrued interest, leading to customer disputes and reconciliation failures. CWE-682 covers incorrect financial calculation; ISO 25010 maintainability requires that the same logical operation produce the same result regardless of call site.

Centralize interest rounding into a single exported function in src/lib/finance.ts and replace every direct Math.round / Math.floor / .toFixed call in financial logic with it.

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

/** Canonical rounding for all interest / APY calculations. */
export function roundInterestCents(value: number): number {
  return new Decimal(value)
    .toDecimalPlaces(0, Decimal.ROUND_HALF_EVEN)
    .toNumber();
}

Search src/ for Math.round, Math.floor, and .toFixed within files that reference interest, apy, or rate, and replace each with roundInterestCents().

• ID: finserv-form-validation.calculation-accuracy.interest-rounding-consistency

• Severity: low

• What to look for: Enumerate all interest/APY calculation sites in the codebase. For each, quote the rounding function used (e.g., Math.round, Math.floor, toDecimalPlaces(2, ROUND_HALF_EVEN)). Count the distinct rounding strategies observed. Report: "X interest calculation sites found using Y distinct rounding strategies."

• Pass criteria: At least 100% of interest and APY calculation sites use the same rounding strategy. The strategy is documented (a constant, comment, or named function like bankersRound). No more than 1 distinct rounding strategy across all interest sites.

• Fail criteria: At least 2 distinct rounding strategies are used across interest calculations, or no rounding strategy is documented.

• Skip (N/A) when: The project has no interest or APY calculations (0 interest calculation sites found).

• Detail on fail: Example: "APY calculation in dashboard uses Math.round() (round-half-up). Interest accrual in API uses Math.floor() (round-down). Inconsistency between display and actual accrual." or "No documented rounding strategy for interest"

• Cross-reference: The bankers-rounding check verifies the correct algorithm is used; this check verifies it is used consistently everywhere.

• Remediation: Document and centralize the rounding strategy in src/lib/finance.ts:

  In src/lib/finance.ts:

  export const ROUNDING_STRATEGY = 'banker'; // round-half-to-even
  
  export function roundInterest(value) {
    // Implement banker's rounding
    const cents = Math.round(value * 100);
    return cents / 100;
  }
  
  // Usage everywhere:
  const interest = roundInterest(principal * rate);