feat(network): 集成ENet库并实现局域网联机对战功能

- 添加ENet库作为网络通信基础，替换原有的原生Socket实现
- 扩展游戏模式支持局域网联机对战（PvP网络模式）
- 重构网络状态结构以适配ENet的Host/Peer模型
- 在图形界面中添加网络对战菜单，支持创建房间和加入房间
- 实现网络消息的发送与接收，包括落子、断开连接等消息类型
- 为网络对战添加定时器轮询机制，实时处理网络事件
- 更新构建系统以编译和链接ENet库

libs/enet/list.c

@@ -0,0 +1,75 @@
+/** 
+ @file list.c
+ @brief ENet linked list functions
+*/
+#define ENET_BUILDING_LIB 1
+#include "enet/enet.h"
+
+/** 
+    @defgroup list ENet linked list utility functions
+    @ingroup private
+    @{
+*/
+void
+enet_list_clear (ENetList * list)
+{
+   list -> sentinel.next = & list -> sentinel;
+   list -> sentinel.previous = & list -> sentinel;
+}
+
+ENetListIterator
+enet_list_insert (ENetListIterator position, void * data)
+{
+   ENetListIterator result = (ENetListIterator) data;
+
+   result -> previous = position -> previous;
+   result -> next = position;
+
+   result -> previous -> next = result;
+   position -> previous = result;
+
+   return result;
+}
+
+void *
+enet_list_remove (ENetListIterator position)
+{
+   position -> previous -> next = position -> next;
+   position -> next -> previous = position -> previous;
+
+   return position;
+}
+
+ENetListIterator
+enet_list_move (ENetListIterator position, void * dataFirst, void * dataLast)
+{
+   ENetListIterator first = (ENetListIterator) dataFirst,
+                    last = (ENetListIterator) dataLast;
+
+   first -> previous -> next = last -> next;
+   last -> next -> previous = first -> previous;
+
+   first -> previous = position -> previous;
+   last -> next = position;
+
+   first -> previous -> next = first;
+   position -> previous = last;
+    
+   return first;
+}
+
+size_t
+enet_list_size (ENetList * list)
+{
+   size_t size = 0;
+   ENetListIterator position;
+
+   for (position = enet_list_begin (list);
+        position != enet_list_end (list);
+        position = enet_list_next (position))
+     ++ size;
+   
+   return size;
+}
+
+/** @} */