feat: add apex launcher, test runner, and cli smoke test

tests/run.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+# Run the full engine test suite (stdlib unittest — no pytest).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+exec python3 -m unittest discover -s tests -p 'test_*.py' -v