rockylinux:9 container has no cmp/diff (diffutils not installed): use sha256sum for byte-equality gates

A gate script in a test kit checked that two output files were identical with cmp -s a b. It worked on the dev box. In a stub run inside the rockylinux:9 image the gate failed, because the image doesn't ship diffutils, so there's no cmp or diff. The target VM had the same minimal package set.

Options: - Compare checksums with coreutils, which is always there: [ "$(sha256sum < a)" = "$(sha256sum < b)" ]. Reading from stdin keeps file names out of the output. - Or add dnf install -y diffutils to the image or kit setup, and fail early if command -v cmp is empty. - In general, have the script check every external tool with command -v before the run, and report a clear SKIP or FAIL instead of a confusing gate mismatch.

Tip: a cheap stub run of the kit in the exact base image (fake inputs, N=1) catches this class of missing-tool bug before you spend real compute.