[510] Add sum checks against values in financial_report_exporter

[Goodnessukaigwe]

Summary

• financial_report_exporter accepted entries whose amounts did not have to equal the report amount, so inconsistent allocations could be serialized as a valid spreadsheet.
• Split totals are now asserted against the base amount using exact bigint arithmetic. A mismatch is FINANCIAL_REPORT_SUM_MISMATCH and is not exported.

Implementation

• assertFinancialReportSplitSum validates every split and the base through validateFinancialAmount, guards digit-limit overflow on the running total, and requires sum(splits) === base.
• validateFinancialReportExporterParams and financial_report_exporter throw on mismatch; exportFinancialReport returns { ok: false, code: SUM_MISMATCH }.
• Omitting entries (or passing an empty array) still emits a single total row, matching previous empty-allocation behavior.

Calculation / serialization behavior

• Comparison is exact (bigint); there is no floating-point tolerance.
• Under-allocation and over-allocation are both rejected.

Validation

• Added __tests__/financial_report_exporter_split_sum.test.ts for exact match, under/over allocation, multiple splits, zero splits, empty entries, and failed async export.
• npm test -- --testPathPatterns=financial_report_exporter, full npm test, npm run lint, and npx tsc --noEmit passed.

Reviewer notes

• Reuses the existing exporter error-code family rather than importing the ledger checker’s split helper, so this change stays isolated from audit_ledger_sum_checker.

Closes #510

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