[CT-09] Implement on-chain Referral Reward Split Calculator in Rust

[devwums]

Problem

Referral reward distribution has no standardised on-chain calculation. Reward splits are computed ad hoc on the backend, making them unauditable and gameable.

Proposed Solution

Create contracts/cntr/referral_reward.rs implementing pub fn calculate_referral_split(total_reward_stroops: i128, referrer_percent: u32) -> Result<(i128, i128), &'static str>. Returns (referrer_amount, referee_amount) where referrer_amount + referee_amount == total_reward_stroops always (any remainder from integer division goes to the referrer). All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/referral_reward.rs
• Returns Err("Referrer percent cannot exceed 100") for referrer_percent > 100
• referrer_amount + referee_amount always equals total_reward_stroops exactly
• Remainder from integer division is added to referrer_amount
• At least 5 unit tests: 50/50, 100/0, 0/100, 70/30, odd total that produces a remainder
• Compiles with cargo test

[BashMan11]

@BashMan11 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi there I would like to take on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @BashMan11 to this issue.

[drips-wave]

Congratulations, @BashMan11! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @BashMan11: 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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[BashMan11]

Resolved in PR #1042. Implementation lives in contracts/cntr/ as specified.

[BashMan11]

Implementation complete. PR: #1042 — will close this issue on merge.