Implement Deterministic Payroll Reconciliation Service with Component-Level Verification

[revatikadam0607]

Summary

Introduces deterministic payroll reconciliation that independently verifies finalized payroll
calculations against stored results. Identifies component-level discrepancies to ensure payroll
integrity and compliance.

Problem

Payroll calculations depend on multiple inputs (attendance, overtime, leave, bonuses, deductions,
tax). Small changes in any stage silently produce different amounts without detection.

Solution

• PayrollDeterminismService: Independently recalculates from input data
• Component-level comparison: Reports first mismatch (not just total difference)
• Decimal.js rounding: Ensures deterministic results (ROUND_HALF_UP)
• Non-destructive verification: Read-only, maintains full audit trail
• Tolerance-based: Allows 1¢ variance for legitimate rounding differences

Components Verified

1. Gross Salary
2. Overtime
3. Bonuses
4. Deductions
5. Tax Components
6. Net Salary

Key Features

✅ Deterministic rounding (Decimal.js precision)
✅ Component-level mismatch reporting
✅ Batch reconciliation support
✅ Read-only verification (no data modification)
✅ Comprehensive audit trail
✅ 25+ test cases covering rounding boundaries
✅ Backward compatible with existing anomaly tracking

Testing

• Rounding boundary tests (.005 edge cases)
• Component mismatch detection for each type
• Tolerance and variance calculations
• Edge cases (zero values, large amounts, missing data)
• Determinism verification (identical inputs = identical outputs)

Database Changes

• Payroll model: Add input metadata fields and components object
• New PayrollReconciliation collection: Track verification status

Acceptance Criteria Met

✅ Finalized payroll can be independently reconciled
✅ Component-level differences are reported
✅ No payroll data modification
✅ Identical inputs produce identical results
✅ Deterministic rounding behavior
✅ Tests cover rounding boundaries and mismatches

Related Issue

Closes#1990 - Payroll Calculation Determinism & Reconciliation

@Dev1822

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