feat(cntr): implement dispute window checker and cancellation policy enforcer

[RennyThompson]

Summary

Resolves two open issues by adding pure-Rust utility modules to contracts/cntr/src/.

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[CT-05] Dispute Window Checker — closes #1022

File: contracts/cntr/src/dispute_window.rs

• is_within_dispute_window(checkout_timestamp, current_timestamp, window_seconds) -> bool
• dispute_window_expires_at(checkout_timestamp, window_seconds) -> u64
• Default constant: DEFAULT_DISPUTE_WINDOW_SECONDS = 172_800 (48 h)
• 5 unit tests: inside window, outside window, exactly at boundary, at checkout moment, expiry timestamp

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[CT-16] Cancellation Policy Enforcer — closes #1033

File: contracts/cntr/src/cancellation_policy.rs

• CancellationPolicy enum deriving Debug, PartialEq, Clone with variants Flexible, Moderate, Strict
• apply_policy(policy, hours_before_start, amount_stroops) -> i128
  • Flexible: always 100% refund
  • Moderate: 100% ≥ 48 h, 50% 24–47 h, 0% < 24 h
  • Strict: 100% ≥ 96 h, 0% otherwise
• Integer arithmetic only, no floating point
• 9 unit tests (3 per policy)

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Other

• Cleaned up duplicate pub mod entries in lib.rs and registered both new modules.

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