feat: add receipt registry with getReceiptByTxHash lookup (fixes #613)

[s6pa1rta3n-lab]

Summary

This PR implements a local receipt registry with transaction hash lookup for the Stellar Split SDK, resolving issue #613.

Acceptance Criteria & Implementation Checklist

• registerReceipt(receipt: PaymentReceipt): void exported — stores receipt keyed by receipt.txHash (overwriting on duplicate hash)
• getReceiptByTxHash(txHash: string): PaymentReceipt | null exported — returns stored receipt or null if not found
• getAllReceipts(): PaymentReceipt[] exported — returns all registered receipts ordered chronologically by receipt.timestamp ascending without mutating internal store
• clearReceipts(): void exported for test teardown (idempotent)
• Registry is module-level (singleton) — not tied to any class instance
• All functions exported from src/index.ts
• Comprehensive unit tests in test/receipt.registry.test.ts covering registration, hash lookup, missing hash handling, timestamp ordering, and clearing

Closes #613

Verification

• Unit tests in test/receipt.registry.test.ts passing 10/10 tests
• All existing receipt tests passing 30/30 tests

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC