Validate hardlink fallback logic on NTFS (via WSL or mounted drives)

[Rakshat28]

Many users will run bdstorage via Windows Subsystem for Linux (WSL) targeting their Windows C:\ drive (NTFS), or mount external NTFS hard drives in Linux. NTFS does not support Linux CoW reflinks, so our primary deduplication strategy will fail.

Acceptance Criteria:

• Test bdstorage dedupe on an NTFS-formatted mount.
• Assert that the engine correctly identifies that reflinks are impossible and prompts the user/fails cleanly unless --allow-unsafe-hardlinks is provided.
• Assert that the hardlink fallback successfully works across NTFS boundaries.