[502] Verify audit_ledger_sum_checker math using detailed numeric test data

[Goodnessukaigwe]

Summary

• audit_ledger_sum_checker already implemented overflow-safe sums, split assertions, and round-half-to-even helpers, but Jest coverage did not pin multi-entry fixtures with independently derived expected totals.
• Adds a dedicated numeric suite so reviewers can audit the arithmetic from the test file without relying on the implementation as the source of truth.

Implementation

• Tests only; no production behavior change.
• Fixtures cover mixed string/number/bigint ledger entries, zeros, stroop-scale decimals as integers, three-way split reconstruction, shortfalls, exact and inexact division, remainder-preserving divideWithRounding, and fee-style applyRoundedScale cases that distinguish half-even from half-up.

Calculation / serialization behavior

• Expected values are integer minor units computed by hand (for example 15_000_000 + 2_500_000 + 4_000_000 = 21_500_000; 5/2 rounds to 2, not 3).

Validation

• npm test -- --testPathPatterns=audit_ledger_sum_checker, full npm test, and npx tsc --noEmit passed.

Reviewer notes

• Rate-limit buckets are reset in beforeEach because sumLedgerAmounts records calls.

Closes #502

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