Root cause of #49145: the Windows ZIP-update path did rmtree(dst) then
copytree(src, dst). If the copy failed partway — common on that path,
which only runs because file I/O is already flaky on the machine — the
directory was left deleted with nothing copied back. ui-tui/ vanishing
is what broke 'hermes --tui' (WinError 267), but the bug hit every
top-level directory.
_atomic_replace_dir stages the new copy into a sibling temp dir and only
swaps it in on full success, restoring the original on failure. A failed
update now leaves the live tree untouched instead of half-deleted.
