feat(graders): implement medium_grader — 70% success rate + 30% strategy diversity

graders/medium_grader.py

@@ -3,5 +3,17 @@ Medium Grader — Success + Diversity
 Evaluates breakthrough and variety of strategies used.
 """
 def grade_medium(history: list[dict]) -> float:
-    """Scaffold for medium grader."""
-    return 0.0
+    if not history:
+        return 0.0
+    
+    # Success (70%)
+    success_rates = [h.get("attack_success_estimate", 0.0) for h in history]
+    success_score = max(success_rates)
+    
+    # Strategy Diversity (30%)
+    strategies = set([h.get("strategy_type") for h in history])
+    unique_count = len(strategies)
+    diversity_score = min(1.0, unique_count / 5.0)
+    
+    total = (0.7 * success_score) + (0.3 * diversity_score)
+    return round(min(1.0, max(0.0, total)), 4)