fix NaN panics and eliminate redundant graph rebuilds

- All partial_cmp().unwrap() → unwrap_or(Ordering::Equal) to prevent
  NaN panics in sort operations across neuro.rs, graph.rs, similarity.rs
- replay_queue_with_graph: accepts pre-built graph, avoids rebuilding
  in agent_prompt (was building 2-3x per prompt)
- differentiate_hub_with_graph: same pattern for differentiation
- Simplify double-reverse history iteration to slice indexing

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/similarity.rs

@@ -100,7 +100,7 @@ pub fn pairwise_similar(
         }
     }
 
-    results.sort_by(|a, b| b.2.partial_cmp(&a.2).unwrap());
+    results.sort_by(|a, b| b.2.partial_cmp(&a.2).unwrap_or(std::cmp::Ordering::Equal));
     results
 }