Implement invoice payment pooling

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Processing payments across many invoices individually wastes Stellar transaction fees. This issue adds pool_pay() accepting a Vec of (invoice_id, amount) pairs — all token transfers are batched into the minimum number of contract calls, reducing total fee cost for multi-invoice payers.

Technical Context

Involves lib.rs — new pool_pay(env, payer: Address, payments: Vec<(u64, i128)>). Require payer auth once. Compute total amount across all payments. Transfer total from payer to contract in one token_client.transfer(). Then distribute to each invoice using internal accounting without additional token transfers.

Acceptance Criteria

• Single token transfer from payer regardless of number of invoices
• Each invoice funded updated correctly
• Any invalid payment (wrong status, over limit) reverts entire call
• Auto-release triggers on any invoice that becomes fully funded
• Test pools 3 invoice payments and verifies all funded amounts correct
• All existing cargo tests pass
• cargo clippy passes with zero warnings

---

[james2177]

@james2177 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, i would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @james2177 to this issue.

[drips-wave]

Congratulations, @james2177! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @james2177: 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 🌊