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/graph.rs

@@ -180,7 +180,7 @@ impl Graph {
         let n = degrees.len();
         if n < 2 { return 0.0; }
 
-        degrees.sort_by(|a, b| a.partial_cmp(b).unwrap());
+        degrees.sort_by(|a, b| a.partial_cmp(b).unwrap_or(std::cmp::Ordering::Equal));
         let mean = degrees.iter().sum::<f64>() / n as f64;
         if mean < 1e-10 { return 0.0; }
 
@@ -448,7 +448,7 @@ fn label_propagation(
 
             // Adopt the label with most votes
             if let Some((&best_label, _)) = votes.iter()
-                .max_by(|a, b| a.1.partial_cmp(b.1).unwrap())
+                .max_by(|a, b| a.1.partial_cmp(b.1).unwrap_or(std::cmp::Ordering::Equal))
             {
                 let current = labels[key];
                 if best_label != current {
@@ -670,7 +670,7 @@ Categories: core={core} tech={tech} gen={gen} obs={obs} task={task}",
     // Show history trend if we have enough data points
     if history.len() >= 3 {
         report.push_str("\n\nMetrics history (last 5):\n");
-        for snap in history.iter().rev().take(5).collect::<Vec<_>>().into_iter().rev() {
+        for snap in &history[history.len().saturating_sub(5)..] {
             report.push_str(&format!("  {} — σ={:.1} α={:.2} gini={:.3} cc={:.4} fit={:.3}\n",
                 snap.date, snap.sigma, snap.alpha, snap.gini, snap.avg_cc, snap.avg_schema_fit));
         }

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
 }