Document ACL, xattr, sparse-file, and hardlink backup behavior (#19)

* Document ACL, xattr, sparse-file, and hardlink backup behavior

Extend the README Limitations section with the filesystem-feature
behaviors that weren't documented: ACLs/xattrs/flags are never
preserved by either backend, sparse files are materialized as full
allocated copies, source-side hardlinks are not preserved as
hardlinks in the backup, and ownership is only preserved when
running as root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Soften README timestamp/ownership claims for external-rsync backend

Nanosecond mtime precision and 'silently logged chown failures' are
Python-backend specifics that don't hold for --external-rsync: rsync's
own timestamp precision depends on the build/filesystem, and it skips
disallowed ownership changes itself rather than going through our
logging path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

IMPROVEMENTS.md

@@ -138,7 +138,7 @@ Acceptance criteria:
 
 ### Define filesystem compatibility
 
-- [ ] Document behavior for ACLs, xattrs, flags, sparse files, source hardlinks,
+- [x] Document behavior for ACLs, xattrs, flags, sparse files, source hardlinks,
   ownership, permissions, and timestamps.
 - [x] Add an explicit one-filesystem policy.
 - [ ] Make Python and external implementations provide equivalent guarantees or