test(audit-guard): wire drift_validator_test.rs into crate; drop unused FixedOffset

[Paranoa-dev]

Test: wire drift_validator_test.rs into the crate (GrantFox OSS / Third Campaign)

Problem

drift_validator_test.rs existed but was never compiled or run:

• grep -rn drift_validator_test . matched only the file's own first line.
• It was not declared with mod drift_validator_test; anywhere in lib.rs (module
  list at lines 6‑8, 24‑32, 154), and it was not under tests/.

As a result, validate_drift and DriftEvent::validate_structure had zero executed
coverage despite a test file existing. Additionally, drift_validator.rs:33 imported
FixedOffset from chrono, which is unused (only DateTime is used).

Changes

• Wired the test in at the correct scope. The test module uses super::* and
  references DriftEvent, validate_drift, and DriftError. Those names resolve
  only inside the drift_validator module (where DriftError is imported via
  use crate::drift_error::DriftError;), so the declaration was added inside
  drift_validator.rs rather than at the crate root:

  #[cfg(test)]
  mod drift_validator_test;

• Removed the unused import. use chrono::{DateTime, FixedOffset}; →
  use chrono::DateTime;, clearing the unused-import warning.

Verification

• cargo test --locked (in src/audit-guard) now compiles and runs the drift
  validator tests:
  • test_valid_drift
  • test_exceeds_threshold
  • test_malformed_id
  • test_invalid_timestamp
• The crate builds with no unused-import warnings from drift_validator.rs.

Files changed

• src/audit-guard/src/drift_validator.rs

closes #349