readThemeFileGuarded relied on O_NOFOLLOW to reject a symlinked theme file.
Windows has no such flag: the constant is undefined, OR-ing it in is a
no-op, and the open followed the link, so a symlink planted in the themes
directory published whatever it pointed at. Check with lstat first where
the flag is missing; the descriptor-bound checks after the open still
narrow what a swap in that window can reach.

Co-Authored-By: Claude Code <noreply@anthropic.com>