[CT-40] Add a shared common_types crate for all sandbox contracts

[yusuftomilola]

Overview

Create a shared contracts/sandbox/common_types/ crate that all sandbox contracts can import for shared primitives (e.g., a TimestampRange struct, a bps_to_amount helper). This reduces code duplication across contracts.

Acceptance Criteria

• contracts/sandbox/common_types/Cargo.toml is created as a library crate
• Defines at least: TimestampRange { start: u64, end: u64 } with a is_valid() method
• Defines bps_to_amount(total: u128, bps: u32) -> u128 helper
• At least two sandbox contracts import and use this crate
• All affected contracts still compile with cargo build

[TheDEV111]

@TheDEV111 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi, I have open source contribution experience and I'm vast i SC design, pls can I have this issue

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[xeeenab]

@xeeenab has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi, I’d like to work on implementing this feature and have reviewed the requirements. I’ll ensure it integrates cleanly with the existing codebase and follows the project’s guidelines, please let me know if there are any specific preferences before I begin.

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[drips-wave]

Congratulations, @xeeenab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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[drips-wave]

This issue has been completed by @xeeenab as part of the Stellar Wave Program's 4th Wave 🥳

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