feat(dedup): add generateIdempotencyKey and canonical key registry (closes #612)

[s6pa1rta3n-lab]

Description

Closes #612.

This PR implements canonical payment idempotency key generation and in-memory key registration in src/dedup.ts, exported via src/index.ts.

Changes

• Implemented generateIdempotencyKey(params: { invoiceId: string; payer: string; amount: bigint; nonce?: string }): string using Node.js built-in crypto.createHash('sha256').
• Implemented isKnownKey(key: string): boolean, registerKey(key: string): void, and clearKeys(): void backed by an in-memory Set.
• Exported all new functions and types from src/index.ts.
• Added comprehensive unit tests in test/dedup.test.ts covering determinism, collision resistance across parameter variations, nonce handling, key registration, teardown reset, and root exports.

Acceptance Criteria Checklist

• generateIdempotencyKey(params: { invoiceId: string; payer: string; amount: bigint; nonce?: string }): string exported from src/dedup.ts
• Key is a deterministic hex string: SHA-256 of "{invoiceId}:{payer}:{amount}" (with optional :{nonce} suffix when provided)
• Same inputs always produce the same key (deterministic); different inputs produce different keys with high probability
• isKnownKey(key: string): boolean and registerKey(key: string): void exported — backed by an in-memory Set within the module
• clearKeys(): void exported for test teardown
• All exported from src/index.ts
• Unit tests: same params → same key, different amount → different key, nonce changes key, isKnownKey after registerKey, clearKeys resets state

Payout Routing

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