feat(dedup): implement generateIdempotencyKey with canonical SHA-256 and key registry (#612)

[s6pa1rta3n-lab]

Summary of Changes

• Implements generateIdempotencyKey(params: IdempotencyParams): string in src/dedup.ts using Node.js built-in crypto.createHash('sha256').
• Canonicalizes parameters into "{invoiceId}:{payer}:{amount}" or "{invoiceId}:{payer}:{amount}:{nonce}" to produce deterministic 64-character hex digests.
• Implements in-memory key registration and checking via isKnownKey(key) and registerKey(key) backed by an internal module Set.
• Provides clearKeys() for clean test teardown and resets.
• Exports all deduplication utilities and IdempotencyParams from src/index.ts.
• Adds unit tests in test/dedup.test.ts verifying key determinism, differentiation on varying amounts/invoices/payers/nonces, key registration, teardown reset, and preservation of existing Deduplicator class behavior.

Closes #612
/claim #612

Payout Routing

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