Document Python/external-rsync backend differences (#20)

* Document Python/external-rsync backend differences

- Note that mixing --external-rsync (which can recreate device/socket
  entries via --archive's -D) with the default Python hardlink-forward
  step is unsafe: _recursive_hardlink() only understands files,
  symlinks, and directories and errors on anything else.
- Note that --dry-run is I/O-free with --external-rsync but still
  fully copies into a throwaway staging directory with the Python
  backend, though neither leaves a lasting change.

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

* Clarify external-rsync dry-run description

Copilot review feedback on PR #20: rsync's --dry-run still reads and
stats source files to compute what would change, it just avoids
writing to the destination - "performs no I/O" overstated that.

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

@@ -141,7 +141,7 @@ Acceptance criteria:
 - [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
+- [x] Make Python and external implementations provide equivalent guarantees or
   document every difference.
 - [ ] Test supported behavior on Linux and macOS.