Add governance proposal salt to prevent vote-replay across re-submitted proposals

[greatest0fallt1me]

Description

A proposal in governance.rs is keyed by an incrementing id, but identical re-submitted payloads can be voted on by re-using off-chain signatures. Add a per-proposal salt: BytesN<32> baked into the signed vote message so signatures are bound to a specific proposal instance.

Requirements and Context

• Generate salt at proposal creation using Env::prng
• Include salt in canonical vote message
• Reject signatures whose salt does not match
• 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/governance-vote-salt

2. Implement changes
   • contracts/predictify-hybrid/src/governance.rs
   • contracts/predictify-hybrid/src/governance_tests.rs
3. Test and commit
   • cargo test -p predictify-hybrid governance_salt
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

feat: bind governance vote signatures to per-proposal salt

Acceptance Criteria

• Signature for one proposal is rejected on another with same payload
• Salt is stored and exposed via view
• Salt entropy comes from Env::prng, not block timestamp

Guidelines

• Validate auth, overflow, and storage TTL/archival
• Clear documentation and inline comments
• Timeframe: 96 hours

[CodeNinjaStephen]

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

Good day maintainer, Im interested on this issue and I’ll appreciate the opportunity to assist on this issue.

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

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