feat: implement issues #870-#873 (Drips Wave)

[ahmadrabiumustapha]

Issue #873 - releasePartial with basis-point validation

• Add InvalidBpsError (code: INVALID_BPS) for bps outside 1-10000
• Add OverReleaseError (code: OVER_RELEASE) when total would exceed 10000 bps
• Add PartialReleaseResult type { txHash, bps, amountReleased, remaining, totalReleasedBps }
• Implement StellarSplitClient.releasePartial(invoiceId, bps) wrapping release_partial

Issue #872 - cancelInvoice with pre-payment guard

• Add CannotCancelFundedInvoiceError (code: CANNOT_CANCEL_FUNDED_INVOICE) with paidAmount
• Implement StellarSplitClient.cancelInvoice(invoiceId) with funded check before any RPC write
• Emit invoice:cancelled event on success; add event type to SplitClientEventMap

Issue #871 - bumpInvoiceTtl and getTtl

• Add InvoiceTerminatedError (code: INVOICE_TERMINATED) for terminal-state invoices
• Add TtlInfo type { ttlLedgers, approximateDays, health: TtlHealth }
• Add TtlHealth union type ('healthy' | 'warning' | 'critical')
• Implement StellarSplitClient.bumpInvoiceTtl(invoiceId) - refuses terminal invoices
• Implement StellarSplitClient.getTtl(invoiceId) - returns health-classified TtlInfo

Issue #870 - getFundingVelocity with hourly bucket aggregation

• Add VelocityBucket type { hour, timestamp, amount }
• Implement StellarSplitClient.getFundingVelocity(invoiceId, opts?) - last 168h default
• Export computeTrendingScore(buckets) standalone pure function (0-100 score)
• Implement StellarSplitClient.isTrending(invoiceId) - score > 70 threshold

All new symbols exported from src/index.ts.
Tests added in test/drips-wave-issues.test.ts.
closes #870
closes #871
closes #872
closes #873

[drips-wave]

@ahmadrabiumustapha Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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