Add buildPaymentMemo and parsePaymentMemo helpers to memoBuilder

[Kingsman-99]

Description

src/memoBuilder.ts builds transaction memos but has no standardised helper for the specific memo format used on invoice payment transactions. Different callers construct the memo string ad-hoc, causing inconsistency in Horizon transaction history. This issue adds a single canonical builder.

Acceptance Criteria

• 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, round-trip, non-split memo returns null

Context

• Target files: src/memoBuilder.ts, src/index.ts
• Stellar memo text limit is 28 bytes — use Buffer.byteLength(str, 'utf8') to measure

[bobo-24]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[Olisachukwuma1]

hello please i would like to work on this issue i have experience as i am a seasoned softwareengineer

[grantfox-oss]

🦊 GrantFox — @Olisachukwuma1 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #610)
2. Your PR will be reviewed by the Stellar-split maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@Olisachukwuma1's PR #707 was approved and merged by @Kingsman-99.

🏆 @Olisachukwuma1: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (414 total points). Track your full progress on GrantFox.

👏 Great work, @Olisachukwuma1! Keep contributing to Stellar-split.