CVE-2026-43619: fix symlink-race TOCTOU on receiver-side path-based
syscalls by routing chmod/chown/rename/mkdir/symlink/mknod/link/
unlink/rmdir/utimensat/stat/lstat/open through symlink-race-safe
do_*_at() wrappers, securing change_dir() chdir-escape and
copy_file source/dest opens; harden secure_relative_open() to
confine basedir resolution and reject all "../" components