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Add a rootless unit test that reaches the actual st_dev skip branch in both the sequential and parallel (-m) scanners: a symlink nested under the scan root points at a directory on /dev/shm (a different device than the build fs) and, under --copy-links semantics, -x must drop that subtree while a plain scan includes it. Skips only when no cross-device target exists. Integration OneFileSystem test now cleans both dest dirs up front and reports a busy test mountpoint instead of ignoring the umount result. RSYNC_COMPAT.md notes that cross-filesystem mount-point subdirectories are dropped entirely (rsync parity).