The explainer smoke test's 30 s timeout is reached when the suite runs on every core #1264

Where: tests/test_explainer_code_smoke.py.

Current gap: test_runnable_explainer_python_blocks[differential-privacy.md-python-block-1] takes about 5 s alone but timed out at 30 s once when the whole suite ran with -n auto on a busy machine, which makes a parallel make test-fast flaky.

Suggested approach: Scale the timeout with the machine load (or retry a timed-out block once with the slow timeout) and report the elapsed time in the failure.

Done when: ten parallel runs on a loaded machine pass, and a real hang still fails with the explainer and block named.