ci: add NTFS hardlink fallback validation

[KGFCH2]

Addresses #34.

Summary

This PR adds automated validation for NTFS storage environments, improving compatibility for WSL users and systems using external NTFS-formatted drives.

Key Changes

Hardlink Fallback Testing

• Added test_ntfs_hardlink_fallback to tests/integration_tests.rs.
• Validates that bdstorage correctly handles environments where reflinks are unavailable by:
  • Safely skipping unsupported operations, or
  • Falling back to hard links when explicitly enabled.

Cross-Boundary Verification

• Verified that hard links are correctly created and detected across the NTFS mount within the Linux environment.

NTFS CI Job

• Added a dedicated ntfs-test job to ci.yml that:
  • Provisions an NTFS loopback device.
  • Executes integration tests against the mounted NTFS filesystem.
  • Ensures fallback deduplication logic is continuously validated in CI, regardless of the host runner’s primary filesystem.

Result

This update significantly improves the reliability and portability of bdstorage for users operating in mixed-OS setups, WSL environments, and legacy filesystem configurations.