Fix MIP fallback return values: ensure consistent unpacking

mip.py

@@ -43,7 +43,8 @@ def design_with_mip(
         )
         from main import design_with_mst
 
-        return design_with_mst(turbines, substation)
+        connections = design_with_mst(turbines, substation)
+        return connections, turbines
 
     if get_max_capacity_func:
         max_mw = get_max_capacity_func(cable_specs, voltage, power_factor)
@@ -103,7 +104,8 @@ def design_with_mip(
         # 返回默认方案，如 MST
         from main import design_with_mst
 
-        return design_with_mst(turbines, substation)
+        connections = design_with_mst(turbines, substation)
+        return connections, turbines
 
     # 提取结果
     cluster_assign = [-1] * n_turbines