feat(contracts): add cntr crate — grace period handler & payment validator

[devbackend513-ux]

Summary

Implements the contracts/cntr crate resolving both issues.

[CT-08] Subscription Grace Period Handler (closes #1025)

• contracts/cntr/src/grace_period.rs
• SubscriptionStatus enum deriving Debug, PartialEq with variants Active, InGracePeriod, Expired
• get_subscription_status(expiry_ts, current_ts, grace_seconds) -> SubscriptionStatus
  • Active when current_ts < expiry_ts
  • InGracePeriod when expiry_ts <= current_ts < expiry_ts + grace_seconds
  • Expired when current_ts >= expiry_ts + grace_seconds
• Default grace period constant: DEFAULT_GRACE_SECONDS = 259_200 (3 days)
• 6 unit tests covering all 3 states and exact boundary values

[CT-18] Payment Verification Test Suite (closes #1035)

• contracts/cntr/src/payment_validator.rs
• validate_payment(paid_amount, expected_amount, tolerance) -> Result<(), PaymentError>
• PaymentError enum: NegativePayment, ZeroPayment, Underpayment
• contracts/cntr/tests/payment_verification_tests.rs — 11 integration tests:
  • exact amount, overpayment, within tolerance, one stroop below tolerance
  • underpayment well below, zero payment, negative payment
  • zero expected amount, tolerance of zero (pass + fail), large i128 value

Files Changed

contracts/Cargo.toml                              — added cntr to workspace members
contracts/cntr/Cargo.toml                         — new crate manifest
contracts/cntr/src/lib.rs                         — module declarations
contracts/cntr/src/grace_period.rs                — CT-08 implementation + tests
contracts/cntr/src/payment_validator.rs           — CT-18 implementation
contracts/cntr/tests/payment_verification_tests.rs — CT-18 test suite (11 tests)

Testing

All tests pass with cargo test -p cntr. CI workflow (.github/workflows/CI.yaml) runs cargo test --all which will validate this crate.

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