Implement Probabilistic Logic Networks (PLN) inference rules:
- TruthValue struct with clamped strength/confidence
- deduction(): chain two implications A->B and B->C
- revision(): merge two independent truth estimates
- negation(): logical NOT (inverts strength, preserves confidence)
- conjunction(): logical AND (multiply strength, min confidence)
- score_with_evidence(): combine base score with confirmations and contradictions
10 unit tests covering basic operations, boundary cases,
symmetry, zero-confidence handling, and output bounds.
Part of #29
