refactor: 重构护盾机制为数值减免型

1. 调整护盾逻辑：从单次抵挡改为按伤害量减免
2. 更新配置、注释与UI提示文本，统一术语表述
3. 修复prefab布局适配新的护盾显示样式

assets/resources/game/heros/top.prefab

@@ -1277,7 +1277,7 @@
         "__id__": 65
       }
     ],
-    "_active": false,
+    "_active": true,
     "_components": [
       {
         "__id__": 73
@@ -1347,7 +1347,7 @@
     },
     "_lpos": {
       "__type__": "cc.Vec3",
-      "x": -25,
+      "x": -20,
       "y": 0,
       "z": 0
     },
@@ -1388,8 +1388,8 @@
     },
     "_contentSize": {
       "__type__": "cc.Size",
-      "width": 50,
-      "height": 10
+      "width": 40,
+      "height": 11
     },
     "_anchorPoint": {
       "__type__": "cc.Vec2",
@@ -1691,8 +1691,8 @@
     },
     "_contentSize": {
       "__type__": "cc.Size",
-      "width": 50,
-      "height": 10
+      "width": 40,
+      "height": 11
     },
     "_anchorPoint": {
       "__type__": "cc.Vec2",
@@ -1813,7 +1813,7 @@
         "__id__": 99
       }
     ],
-    "_active": true,
+    "_active": false,
     "_components": [
       {
         "__id__": 105