feat: 新增垃圾回收专题 gc-special/gc-deep-dive 50题（单选/填空/判断/多选/简答各10）

topics/index.json

@@ -1,6 +1,6 @@
 {
   "version": "1.0.0",
-  "updated": "2026-09-02",
+  "updated": "2026-09-03",
   "topics": [
     {
       "slug": "qunar-ai-fullstack",
@@ -197,6 +197,29 @@
           }
         }
       ]
+    },
+    {
+      "slug": "gc-special",
+      "name": "垃圾回收专题",
+      "description": "垃圾回收（GC）专题：GC 概念与代价、内存碎片与算法、分代 GC、回收器对比、GC Roots、三色标记法",
+      "subtopics": [
+        {
+          "slug": "gc-deep-dive",
+          "name": "GC 综合深入",
+          "description": "垃圾回收专题：GC 概念、内存碎片与算法、分代 GC、回收器对比、GC Roots、三色标记法",
+          "path": "topics/gc-special/gc-deep-dive",
+          "stats": {
+            "total": 50,
+            "by_type": {
+              "single_choice": 10,
+              "fill_blank": 10,
+              "true_false": 10,
+              "multiple_choice": 10,
+              "short_answer": 10
+            }
+          }
+        }
+      ]
     }
   ]
-}
+}