Add a `simulate_deduct(amount)` read-only view that returns the outcome without state change

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. Read-only entrypoint used by clients to pre-flight a deduct without mutating state.

Requirements and Context

• No storage writes
• Returns same struct as deduct
• Authorization not required
• Tested against real deduct
• 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/simulate-deduct

2. Implement changes
   • contracts/vault/src/views.rs (new)
   • contracts/vault/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: read-only simulate_deduct view

Acceptance Criteria

• Read-only
• Returns outcome
• Auth-free
• Parity 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

[IfyJustin91]

@IfyJustin91 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainers,

I'd like to work on this issue. I understand the goal is to add a read-only simulate_deduct view that mirrors the deduct outcome without mutating contract state. I can implement the view, ensure it performs no storage writes or authorization checks, maintain parity with the existing deduct logic, and add comprehensive tests and documentation to satisfy the acceptance criteria.

I'd appreciate being assigned to this issue. Thank you!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @IfyJustin91 to this issue.

[iexwr]

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

I'd like to work on this issue. I will implement a read-only simulate_deduct view that mirrors the deduct outcome without modifying state, requires no authorization, ensures parity with the existing logic, and includes comprehensive tests and documentation.

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

[grantfox-oss]

🦊 GrantFox — @IfyJustin91 has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #511)
2. Your PR will be reviewed by the CalloraOrg maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

Congratulations, @IfyJustin91! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @IfyJustin91: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊