Add Kani harness for vault.transfer correctness under adversarial input

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. Add a Kani proof harness asserting that transfer preserves total supply and respects authorization.

Requirements and Context

• Kani harness file
• CI workflow runs Kani on PRs
• Proof passes
• Doc explains harness
• 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 task/kani-transfer

2. Implement changes
   • contracts/vault/proofs/transfer.rs (new)
   • .github/workflows/kani.yml (new)
   • docs/formal-verification.md (new)
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

test: Kani proof harness for vault.transfer

Acceptance Criteria

• Harness compiles
• CI runs
• Proof passes
• Doc explains

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

[ZuLu0890]

@ZuLu0890 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ZuLu0890 to this issue.

[drips-wave]

Congratulations, @ZuLu0890! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @ZuLu0890: 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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[grantfox-oss]

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

@ZuLu0890's PR #523 was approved and merged by @greatest0fallt1me.

🏆 @ZuLu0890: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (716 total points). Track your full progress on GrantFox.

👏 Great work, @ZuLu0890! Keep contributing to CalloraOrg.

[drips-wave]

This issue has been completed by @ZuLu0890 as part of the Stellar Wave Program's 6th Wave 🥳

😎 @ZuLu0890: You earned 100 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. You can also Leave a review to share your experience working on this issue.

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