Fix MIP toggle bug: handle PuLP import gracefully

main.py

@@ -14,11 +14,7 @@ from sklearn.cluster import KMeans
 
 from esau_williams import design_with_esau_williams
 from ga import design_with_ga
-
-try:
-    from mip import design_with_mip
-except ImportError:
-    design_with_mip = None
+from mip import design_with_mip
 
 # 设置matplotlib支持中文显示
 plt.rcParams["font.sans-serif"] = ["Microsoft YaHei", "SimHei", "Arial"]
@@ -1715,7 +1711,7 @@ def compare_design_methods(
                 f"  [GA] Cost: ¥{eval_ga['total_cost']:,.2f} | Loss: {eval_ga['total_loss']:.2f} kW | Circuits: {n_circuits_ga}"
             )
 
-        if use_mip and design_with_mip:
+        if use_mip:
             # --- Run 5: Mixed Integer Programming ---
             mip_name = f"{name} (MIP)"
             conns_mip, turbines_mip = design_with_mip(