Address review: replace bare KeyError with a contextual ValueError invariant
check in _grade_unit (fail-loud, no catch/skip/fallback); tighten
_drop_orphan_pairs to require exactly one original + one mirror with no extra
illegal-role records (total==2); clarify InferenceResult docstring to unit-grained
semantics. Add tests for missing-prediction descriptive error and extra-illegal-role
pair drop.
iomgaa
8a69a54078