feat(audit): integrate Nirium audit trail anchoring and verification for payouts

[Santia2004]

Summary

Integrates an independent, domain-separated audit trail adapter for Fundable payout and stream settlements using Nirium's Audit Trail framework.

Changes

• Added canonical settlement schema FundablePayoutAuditRecord.
• Implemented deterministic canonical SHA-256 hashing (computeCanonicalAuditHash).
• Added domain-separated attestation signing with prefix nirium-audit-v1:<sha256>.
• Added anchorFundablePayout for external IPFS anchoring without modifying Soroban contracts or custody logic.
• Implemented verifyAnchoredPayoutReceipt for independent verification by finance teams and external auditors.
• Added comprehensive unit tests (src/audit/adapter.test.ts) and runnable verification example (examples/verify-anchored-payout.ts).

Verification

• pnpm test: All unit tests passing.
• pnpm typecheck: Type check clean.
• pnpm verify: Full SDK pipeline passed.

[coderabbitai]

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Run ID: de3c89fe-7834-4e9a-b7cc-9065dcd69f8f

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[Santia2004]

team! Just following up on this PR. It's fully ready for review with clean tests and no merge conflicts. Whenever you have a moment, could you please approve the workflow run and take a look? Let me know if you need any adjustments.