Add events.rs replay protection nonce in EventEmitter to dedup retried emissions

[greatest0fallt1me]

Description

Cross-contract retries or batch fallbacks can cause EventEmitter to emit the same logical event twice. Add a monotonic per-(emitter, topic) nonce so consumers can dedupe at-most-once even if upstream retries fire repeatedly.

Requirements and Context

• Nonce stored under DataKey::EventNonce(topic)
• Embedded in the event payload as nonce: u64
• Reset only on contract upgrade (documented)
• 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/event-emitter-replay-nonce

2. Implement changes
   • contracts/predictify-hybrid/src/events.rs
   • New test file under tests/
3. Test and commit
   • cargo test -p predictify-hybrid event_replay_nonce
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

feat: add per-topic replay nonce to EventEmitter

Acceptance Criteria

• Two consecutive emissions on the same topic increment nonce monotonically
• Topic isolation: nonce of topic A does not affect topic B
• Documented behaviour on upgrade

Guidelines

• Clear documentation and inline comments
• Real Soroban SDK idioms in any new helper code
• Timeframe: 96 hours

[laxjovial]

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

Resolving this works for me if you’d please kindly assign. Thanks

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

[drips-wave]

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

🧑‍💻 @laxjovial: 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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