Add a core startup migration that refreshes /exe/self_update_manager.py from the repository copy when the installed runtime updater is missing the socket-safe backup and Desktop cleanup markers.
Validate the source updater before replacing anything, preserve the previous runtime script as a backup, and leave current or non-regular targets untouched. Add regression coverage for stale runtime replacement, safe no-op/refusal paths, and the real repository updater source.
