In-process RsyncHandler couldn't match openrsync protocol 29 flow
after version exchange. Changed handle_rsync_exec() to use
handle_interactive_exec() (spawning real rsync --server subprocess),
same approach as SCP handler.
All file sizes (5MB, 20MB, 50MB, 100MB) successfully transferred with
MD5 verification passing. Transfer speed ~712 KB/s limited by
AES-256-CTR encryption overhead.