Corroborate Python no-test exit-code 5

[hzw0813]

Summary

• stop treating Python test-runner exit code 5 as automatically benign
• require high-confidence exact zero-test observations before suppressing targeted pytest exit 5
• require the paired targeted observer to corroborate opaque pytest/unittest exit 5
• preserve legitimate pytest and unittest no-test behavior while failing closed when a repository-local pytest.py or unittest.py exits 5 before lifecycle observation
• keep evidence classification and evidence-boundary explanation aligned

Why

Both pytest and the stdlib unittest CLI use exit code 5 for no-test outcomes, but the same numeric exit code is not sufficient provenance. Because Python module resolution can execute a repository-local pytest.py or unittest.py, a runner import can terminate with code 5 before ProofDiff observes any test lifecycle. The previous broad exception could therefore erase a real process failure and downgrade verification-failed to a non-failing result.

The fix accepts exit 5 only when exact high-confidence zero-test observer data corroborates the meaning. Targeted pytest can corroborate itself through its observer record; opaque pytest/unittest checks require their paired targeted observer.

Regression coverage

• existing legitimate pytest exit-5 no-collection case remains non-failing
• existing zero-test unittest target remains non-failing
• repository-local pytest.py aborting with SystemExit(5) now fails closed
• repository-local unittest.py aborting with SystemExit(5) now fails closed

Verification

• full npm test passed
• clean npm run clean && npm run build passed
• git diff --check passed
• generated dist refreshed
• temporary updater workflows removed from the final diff