[CT-04] Implement Escrow Refund Amount Calculator with cancellation policy in Rust

[devwums]

Problem

Refund conditions for cancelled bookings are not codified in the contract layer. Refund calculations happen ad hoc on the backend with no on-chain enforceability.

Proposed Solution

Create contracts/cntr/escrow_refund.rs implementing pub fn calculate_refund_amount(booking_amount_stroops: i128, cancellation_hours_before_start: u64) -> i128. Refund rules: 100% refund if cancelled 48h or more before start, 50% refund if 24–47h before, 0% refund if less than 24h before. Integer arithmetic only — no floating point. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/escrow_refund.rs
• Returns booking_amount_stroops for cancellations at or beyond 48 hours
• Returns booking_amount_stroops / 2 (integer division) for 24–47 hours
• Returns 0 for less than 24 hours
• At least 6 unit tests including exact boundary values (24h and 48h)
• No floating-point arithmetic
• Compiles with cargo test

[Userhorlie]

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

I would love to take on this issue.

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

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[Userhorlie]

Resolved in PR #1044. Implementation lives in contracts/cntr/ with all acceptance criteria met and tests passing.

[Userhorlie]

Implemented in PR #1044. All acceptance criteria met, 26 unit tests passing.

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This issue has been completed by @Userhorlie as part of the Stellar Wave Program's 5th Wave 🥳

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