Function gradeEvaluation

Derives a human-readable grade from an argument evaluation result.

Precedence:

1. Evaluation failed (ok=false) → indeterminate
2. Inadmissible assignment → inadmissible
3. Supporting premise false → unsound
4. Counterexample (all supporting true, conclusion false) → counterexample
5. Conclusion vacuously true → vacuously-true
6. Conclusion true → sound
7. Otherwise → indeterminate