仓库组件 + 防御塔升级

- `PlayerInventoryComponent`：玩家库存组件，收集金币/组件/防御塔等道具
- 补全防御塔的升级逻辑，最高 5 级

Assets/GameMain/Scripts/Entity/EntityData/DefenseTowerData.cs

@@ -8,13 +8,16 @@ namespace GeometryTD.Entity.EntityData
     public class DefenseTowerData : EntityDataBase
     {
         [SerializeField] private DefenseTowerStatsData _stats = new DefenseTowerStatsData();
+        [SerializeField] private int _towerLevel = 0;
 
-        public DefenseTowerData(int entityId, int typeId, Vector3 position, Quaternion rotation, DefenseTowerStatsData stats)
+        public DefenseTowerData(int entityId, int typeId, Vector3 position, Quaternion rotation,
+            DefenseTowerStatsData stats, int towerLevel = 0)
             : base(entityId, typeId)
         {
             Position = position;
             Rotation = rotation;
             _stats = stats ?? new DefenseTowerStatsData();
+            _towerLevel = Mathf.Max(0, towerLevel);
         }
 
         public DefenseTowerStatsData Stats
@@ -22,5 +25,11 @@ namespace GeometryTD.Entity.EntityData
             get => _stats;
             set => _stats = value ?? new DefenseTowerStatsData();
         }
+
+        public int TowerLevel
+        {
+            get => _towerLevel;
+            set => _towerLevel = Mathf.Max(0, value);
+        }
     }
 }