feat(truth): add PLN deduction engine (#32)

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

src/truth/mod.rs

@@ -0,0 +1,3 @@
+//! Truth scoring engine — PLN deduction, ECAN attention, and memory scoring.
+
+pub mod pln;