[CT-01] Implement Subscription Expiry Checker functions in Rust

[devwums]

Problem

No standalone helper exists to check whether a subscription has expired. Expiry logic is duplicated across multiple call sites in the contract layer, making it prone to inconsistency.

Proposed Solution

Create contracts/cntr/subscription_expiry.rs implementing two pure functions: pub fn is_subscription_expired(expiry_timestamp: u64, current_timestamp: u64) -> bool and pub fn days_until_expiry(expiry_timestamp: u64, current_timestamp: u64) -> i64 (negative value means expired). No Soroban SDK dependency — pure Rust only. All implementation must live inside contracts/cntr/.

Acceptance Criteria

• File at contracts/cntr/subscription_expiry.rs
• is_subscription_expired(100, 200) returns true
• is_subscription_expired(200, 100) returns false
• is_subscription_expired(100, 100) returns true (expired at boundary)
• days_until_expiry returns negative integers for expired subscriptions
• At least 6 unit tests in a #[cfg(test)] block
• Compiles with cargo test from the contracts/cntr/ directory

[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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[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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