tests/non-root-user.test.mjs locks in both acceptance criteria: a static
assertion that the Dockerfile runtime stage declares a non-root USER (not
root/uid 0, 'USER node' exactly), non-vacuous mutation probes, and a
Docker-gated real-stack probe that starts the stack and asserts 'id -u' and
'id -un' inside the running app container report a non-root user, with the
health endpoint still answering as a regression guard.
