feat(memo): add buildPaymentMemo and parsePaymentMemo helpers (#610)

[s6pa1rta3n-lab]

Description

src/memoBuilder.ts builds transaction memos but lacked a standardised helper for the specific memo format used on invoice payment transactions. This PR adds canonical buildPaymentMemo and parsePaymentMemo helpers and exports them from src/index.ts.

Acceptance Criteria Checklist

• buildPaymentMemo(invoiceId: string, opts?: { tranche?: number }): string exported from src/memoBuilder.ts
• Output format: "split:{invoiceId}" base case; "split:{invoiceId}:t{tranche}" when tranche provided; truncated to 28 bytes (Stellar memo text limit) with no mid-character cut on UTF-8
• parsePaymentMemo(memo: string): { invoiceId: string; tranche?: number } | null exported — returns null for non-split memos
• Round-trip: parsePaymentMemo(buildPaymentMemo(id, opts)) returns the original inputs
• Both exported from src/index.ts
• Unit tests: base format, tranche format, 28-byte truncation, UTF-8 multi-byte / emoji safety, round-trip, non-split memo returns null

Closes #610
/claim #610

Payout Routing

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