The LowCharValiditySignal and HighCharValiditySignal evaluators were already
implemented in classify.rs. All acceptance criteria are met:
- rate < 0.4 → BrokenVector with strength 0.80
- rate > 0.85 → Vector with strength 0.90
- middle band (0.4-0.85) → None
- no text → None
All 80 classification tests pass.
