Add settlement idempotency lock tests

[edehvictor]

Summary

Add tests and guardrails ensuring repeated settlement calls for the same payroll batch cannot pay twice.

Why This Matters

Network retries and user double-clicks should not create duplicate payroll settlements.

Suggested Files

• contracts/src/settlement.rs
• contracts/src/payroll.rs
• contracts/src/storage.rs
• contracts/tests/settlement_idempotency.rs

Tasks

• Store settlement completion markers before or atomically with finalization.
• Reject duplicate settlement attempts.
• Allow safe read-only confirmation of already-settled runs.
• Add retry-focused tests.

Acceptance Criteria

• A payroll batch cannot settle more than once.
• Duplicate execution attempts return a stable already-settled error.
• Confirmation reads remain available.
• Tests cover retry and duplicate submission.

Coordination

Join the Telegram channel for proper coordination: https://t.me/zkpayroll

[pheobeayo]

@pheobeayo has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello, I would like to work on this issue.

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

[TheHalalHunter]

@TheHalalHunter has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer
I’d love to help out with this issue. I’ve checked the current codebase/documentation and know exactly what needs to be updated to make this clearer. I will ensure everything aligns perfectly with your existing contribution guidelines.

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

[drips-wave]

Congratulations, @TheHalalHunter! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @TheHalalHunter: 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.

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[grantfox-oss]

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

@TheHalalHunter's PR #397 was approved and merged by @edehvictor.

🏆 @TheHalalHunter: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (328 total points). Track your full progress on GrantFox.

👏 Great work, @TheHalalHunter! Keep contributing to zkpayroll.

[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @TheHalalHunter as part of the Stellar Wave Program's 8th Wave 🥳

Moderator's reason: [Auto-assessed] The PR was accepted and merged by maintainer @edehvictor (mergedAt: 2026-08-29T17:53:41Z). Furthermore, the contribution successfully adds comprehensive settlement idempotency lock tests as requested in the issue.

😎 @TheHalalHunter: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.