feat(invoice): add split rounding auditor with largest-remainder corr…

[gracepeterfejokwu]

…ection

• Add SplitLine, RoundingAdjustment, and AuditedSplitResult interfaces to src/types.ts
• Implement auditSplitRounding() in src/invoice/rounding.ts:
  • Computes per-recipient floor amounts from bigint total × float ratio
  • Applies largest-remainder method to distribute stroop remainder deterministically (highest fractional part wins, stable tie-break)
  • Records every +/-1 stroop adjustment in AuditedSplitResult.adjustments
  • Throws RoundingOverflowError when total adjustment exceeds ceil(splits.length / 2) stroops
• Add src/invoice/calculator.ts with calculateSplitAmounts() and computeAmounts() wrappers integrating the auditor
• Add 20 tests in test/roundingAuditor.test.ts covering:
  • Exact division (no correction needed)
  • 3-way uneven split requiring +1 stroop correction
  • 1-stroop invoice split across 3 recipients
  • Largest-remainder ordering is deterministic
  • RoundingOverflowError when ratios deviate too far from 1.0
  • Sum invariant across a range of totals and ratios
  • calculator.ts integration

Closes #590

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