Build invoice payment analytics oracle

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Other Soroban contracts need access to StellarSplit payment data without reading raw storage. This issue adds a standardised oracle interface — get_invoice_stats(invoice_id) returns a InvoiceStats struct with funded amount, payment count, unique payer count, and completion percentage — callable by external contracts.

Technical Context

Involves lib.rs — new get_invoice_stats(env, invoice_id: u64) -> InvoiceStats. Define InvoiceStats { funded: i128, total: i128, payment_count: u32, unique_payers: u32, completion_bps: u32 } in types.rs. Compute unique payers by deduplicating invoice.payments payer addresses. No auth required.

Acceptance Criteria

• get_invoice_stats() returns correct funded and total
• payment_count equals total number of payment entries
• unique_payers correctly deduplicates addresses
• completion_bps equals funded * 10_000 / total
• No auth required (read-only oracle interface)
• All existing cargo tests pass
• cargo clippy passes with zero warnings

---

[ahmadadebayo78-boop]

@ahmadadebayo78-boop has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ahmadadebayo78-boop to this issue.

[drips-wave]

Congratulations, @ahmadadebayo78-boop! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @ahmadadebayo78-boop: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊