fix: implement NTFS hardlink fallback and updated warnings

[unpetrifiedstone2005]

This commit adds a fallback to hardlinks when reflinks fail on NTFS/WSL and updates user messaging to guide users toward the correct flags.

Closes #34

[Rakshat28]

we shouldn't hardcode 'NTFS' or 'WSL' into our core error messages. While WSL/NTFS is the most common way users hit this bug, the actual underlying system error is an EXDEV (Cross-device link) failure. This OS-level error occurs whenever a hardlink is attempted across any two different partitions, drives, or filesystems.

To fully resolve Issue #34, we need to address the root architecture:

1. Generic Error Handling: The error messages should indicate a 'Cross-device link' or 'Filesystem boundary' failure, rather than assuming NTFS.
2. Testing: We need a test case that triggers a cross-boundary failure to prove the fallback works.
3. The Vault Location: Currently, vault::vault_root() defaults to ~/.imprint/store. If a user targets an external drive, std::fs::hard_link will always fail. We likely need to discuss modifying the engine to initialize localized .imprint vaults at the root of the target drive.

[Rakshat28]

Closing this PR because we need to address the root EXDEV architectural limitation rather than just updating the error strings