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* Add an explicit one-filesystem policy to backups scantree() gains an opt-in one_filesystem flag: when a subdirectory's device differs from the walk root's (a mount point), its contents are excluded from the walk. The mount-point directory itself is still yielded, so it appears as an empty directory in the backup rather than vanishing silently - matching rsync's --one-file-system behavior. Wired into rsync()'s two source-tree walks so backups never silently pull in content from a bind mount or other filesystem mounted under the source tree. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> * Fix one-filesystem policy: symlinked root and external rsync parity - scantree() computed the walk root's device with os.lstat(), but scandir() follows the root path if it's a symlink. A symlinked root would compute the symlink's own device instead of the target directory's, making every real subdirectory look cross-device and get its contents wrongly excluded. Use os.stat() for the root only. - rsync_ext() (the external-rsync backend) didn't pass --one-file-system, so backups using external_rsync=True still crossed mount points despite the pure-Python path now refusing to. Addresses GitHub Copilot review feedback on PR #18. 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>