Validate OpenZFS block-cloning support and document limitations

[Rakshat28]

ZFS handles deduplication at the block level natively, but OpenZFS 2.2+ recently added support for explicit block cloning (reflinks via copy_file_range). Users on older ZFS versions will see bdstorage fail to reflink. We need to map out exactly how our tool behaves across ZFS versions.

Acceptance Criteria:

• Manually test bdstorage on a system with OpenZFS < 2.2 to verify that it gracefully falls back or errors cleanly with "reflink not supported".
• Manually test on OpenZFS 2.2+ to verify that block cloning succeeds.
• Update README.md with a specific "ZFS Support" section explaining when bdstorage will use block cloning vs. when the user must use --allow-unsafe-hardlinks.