Alfie gf

[alfeedrips]

Summary
Introduces a shared Money fixed-point type for payroll financial math, eliminating JS floating-point drift across tax, benefits, and bonus calculations. Replaces the repeated parseFloat((...).toFixed(7)) + float += anti-pattern with BigInt-based arithmetic that guarantees exact results.
Changes

• backend/src/utils/money.ts (new) — BigInt-based Money class storing values scaled to 7 decimal places (Stellar stroop precision). Supports add, sub, mul, div, percentage, max, isNegative, isZero, toNumber, toFixed.
• backend/src/utils/tests/money.test.ts (new) — 21 tests covering construction, arithmetic, percentage, comparison, and drift regression tests proving gross === totalTax + net invariant preservation.
• backend/src/services/taxService.ts — calculateDeductionsRuleBased and generateReport migrated to Money.
• backend/src/services/benefitsService.ts — generateDraftPayslip migrated to Money for deduction/tax calculations and total accumulation.
• backend/src/services/payrollBonusService.ts — getPayrollRunSummary migrated to Money for amount summation.
  Testing
  npx jest src/services/tests/taxService.test.ts src/utils/tests/money.test.ts --no-coverage
• 39 tests passing (18 taxService + 21 money)
• Drift regression tests verify gross === totalTax + net exactly across 10 and 20 stacked percentage deductions with fractional rates
• All existing taxService tests pass with correct (not just similar) values
  Tradeoffs
• No external dependency — rolled a minimal Money class instead of adding decimal.js or big.js. The class is 76 lines, covers exactly the operations used in the three services, and avoids bundle bloat. If more complex decimal math is needed later, swapping to a library is straightforward since the API surface is similar.
• 7-decimal precision hardcoded — matches Stellar stroop precision. Could be made configurable if other precision requirements emerge.
• toNumber() at boundaries — the services still return number at their public API boundaries (to callers, DB, etc.). The Money class eliminates drift within the calculation; the final toNumber() is a single rounding step, not N accumulated ones.
  Architecture
  backend/src/utils/money.ts ← shared Money type (BigInt, 7 decimals)
  backend/src/services/taxService.ts ← uses Money for deduction rules
  backend/src/services/benefitsService.ts ← uses Money for payslip generation
  backend/src/services/payrollBonusService.ts ← uses Money for summary totals
  Out of scope
• Contract-side (Rust) math — tracked separately (Add property tests for revenue_split dust handling and basis-point rounding #398)
• Non-financial numeric calculations elsewhere in the backend
• Other services not mentioned in the issue
  Closes Financial calculations use floating-point math instead of fixed-point/decimal — cumulative rounding drift #451

[Wilfred007]

Thank you @alfeedrips for your contribution