closes #980 implement reflink (copy-on-write) file copying with fallb…

[romanstingler]

This adds copy-on-write file cloning to improve storage efficiency on supported filesystems.

New reflink.rs module with Linux FICLONE ioctl support.
Updated copy operation to try reflink first, fall back to standard copy.
Works transparently on Btrfs, XFS, Bcachefs - falls back on others, or non-linux OS.

I think checking filesystem type is unreliable because:

Btrfs supports reflink but only on same filesystem
XFS supports reflink but has version requirements
ZFS supports reflink but via different mechanism
ext4 doesn't support reflink at all

In my opinion the cleanest approach is what we're doing:
Attempt the ioctl directly
Let the kernel tell us if it works or not
Clean up and fallback on failure

This is actually the standard approach used by tools like cp --reflink=auto - they don't pre-check, they just attempt and handle the failure gracefully.

/srv/opensource/cosmic/cosmic-files/target/release/tmp master
❯ btrfs filesystem du .
     Total   Exclusive  Set shared  Filename
  73.80MiB       0.00B           -  ./cosmic-files
  73.80MiB       0.00B    61.33MiB  .

tested on BTRFS and EXT4