Add chain-reorg replay protection via ledger sequence high-water mark

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. Track per-developer last-applied ledger sequence and reject any settlement claim from a lower or equal sequence.

Requirements and Context

• high_water_mark stored
• Reject equal/lower seq
• Storage TTL tied to lifecycle
• Test reorg path
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/replay-guard

2. Implement changes
   • contracts/settlement/src/replay_guard.rs (new)
   • contracts/settlement/src/lib.rs
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: ledger-sequence replay protection

Acceptance Criteria

• High-water enforced
• Equal seq rejected
• TTL maintained
• Reorg test passes

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours

[VeronicDev]

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

Hello maintainer I have worked on something like this before and would like yo try it again.bpls assign

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

[drips-wave]

Congratulations, @VeronicDev! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @VeronicDev: 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

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