What: _commit_staged_replacements backs up a regular file as a hardlink or, where links are unsupported, a copy2 to <backup>.tmp + fsync + os.replace to <backup>, then os.replace(staging, dst); only directories move aside. The docstring's 'never absent' claim now holds.
Why: the no-hardlink fallback renamed hermes_bootstrap.py aside before renaming the new one in; a kill between left it absent, so every launcher failed before reaching the journal restore.
Red: pre-fix test_root_files_never_go_missing_mid_swap[False] -> root modules absent mid-swap: hermes_constants.py, hermes_bootstrap.py. Green: both params pass.
8f95df3334