CVE-2026-32711: pydicom has a path traversal in FileSet/DICOMDIR ReferencedFileID allows file access outside the File-set root

A crafted DICOMDIR can set ReferencedFileID to a path outside the File-set root. pydicom resolves the path only to confirm that it exists, but does not verify that the resolved path remains under the File-set root. Subsequent public FileSet operations such as copy(), write(), and remove()+write(use_existing=True) use that unchecked path in file I/O operations. This allows arbitrary file read/copy and, in some flows, move/delete outside the File-set root.