Add payment receipt generator

[Kingsman-99]

Labels: complexity: high

Implement generatePaymentReceipt(invoiceId, payerAddress) that compiles a payment receipt object from on-chain data: invoice details, total paid, timestamps, and a SHA-256 proof hash.

Acceptance criteria:

• Returns { invoiceId, payer, totalPaid, payments[], proofHash, generatedAt }
• proofHash is SHA-256 of invoiceId + payer + totalPaid + ledgerTimestamp
• Works for both completed and in-progress invoices
• Exported with TypeScript types and optional JSON serialisation

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[modrispath]

@modrispath has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'd like to take this on.

I'll implement generatePaymentReceipt(invoiceId, payerAddress) pulling invoice details and payment history from on-chain data, computing the SHA-256 proof hash from the canonical string invoiceId + payer + totalPaid + ledgerTimestamp, and returning the full receipt shape with TypeScript types and optional JSON serialisation.

Works for both completed and in-progress invoices, with tests covering both states plus hash correctness. Ready to start if assigned.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @modrispath to this issue.