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>

README.md

@@ -205,6 +205,24 @@ backups/
 - **Symlinks are preserved as symlinks**, including ones pointing
   outside the source tree - their targets are never followed or
   copied.
+- **ACLs, extended attributes, and filesystem-specific flags (e.g.
+  BSD/macOS `chflags`) are never preserved**, by either implementation.
+  The Python backend never reads or sets them; `--external-rsync` uses
+  `--archive` without `-A`/`-X`/`--fileflags`, so they're dropped too.
+- **Sparse files are not preserved as sparse.** Both implementations
+  do a whole-file copy (`--whole-file` for external rsync), so holes
+  are read and rewritten as allocated zero blocks in the backup.
+- **Hardlinks between source files are not preserved as hardlinks.**
+  Each hardlinked source file is backed up as an independent file;
+  neither backend passes rsync's `-H`. Only hardlinks *within* the
+  backup repository (across snapshots) are meaningful.
+- **Mode and timestamps are always preserved.** The Python backend
+  restores mtime with nanosecond precision; `--external-rsync`'s
+  precision depends on the installed rsync build and destination
+  filesystem. **Ownership (uid/gid) is preserved only when running as
+  root.** In the Python backend a failed `chown` is caught and logged,
+  not fatal; with `--external-rsync`, rsync itself just skips
+  ownership it isn't permitted to set.
 
 ## Development