fix(c-client): 修复 SDK 头文件保护和 demo 编译问题

- SDK main.c: 添加条件编译 #ifndef/#define KENAITO_CONF_CLIENT_IMPL
- Demo main.c: 移除 #include SDK 源文件，改为 extern 声明函数
- 修复 demo 中访问 configs.count 的编译错误

编译方式：
  gcc -o demo main.c ../kenaito-conf-c-client/main.c \
    -I/usr/local/opt/cjson/include -L/usr/local/opt/cjson/lib \
    -lcurl -lcjson -lpthread

kenaito-conf-c-client-demo/main.c

@@ -1,17 +1,10 @@
 /**
  * Kenaito Config Center C Client Demo
  * 
- * 演示如何使用 C 客户端 SDK：
- *   - 初始化客户端
- *   - 注册配置变更回调
- *   - 启动/停止客户端
- *   - 读取配置
+ * 演示如何使用 C 客户端 SDK
  * 
  * 编译：
- *   gcc -o demo main.c -I../kenaito-conf-c-client -L../kenaito-conf-c-client -lkenaito_conf -lcurl -lcjson -lpthread
- * 
- * 或直接编译所有源文件：
- *   gcc -o demo main.c ../kenaito-conf-c-client/main.c -lcurl -lcjson -lpthread
+ *   gcc -o demo main.c ../kenaito-conf-c-client/main.c -I/usr/local/opt/cjson/include -L/usr/local/opt/cjson/lib -lcurl -lcjson -lpthread
  * 
  * 运行：
  *   ./demo
@@ -21,10 +14,39 @@
 #include <stdlib.h>
 #include <string.h>
 #include <signal.h>
-#include "../kenaito-conf-c-client/main.c"
+#include <unistd.h>
+#include <time.h>
+
+/* 声明 SDK 类型和函数 */
+typedef struct {
+    char baseUrl[512];
+    char appName[128];
+    char env[32];
+    char version[32];
+    char token[256];
+    int syncInterval;
+    int ignoreGlobal;
+    char cacheDir[256];
+} Config;
+
+typedef struct {
+    Config config;
+    void *configs;
+    long lastTime;
+    void (*onChange)(const char *key, const char *value);
+    void *curl;
+    int running;
+    void *syncThread;
+} ConfigClient;
+
+extern void config_client_init(ConfigClient *client, Config *config);
+extern const char *config_client_get(ConfigClient *client, const char *key);
+extern void config_client_start(ConfigClient *client);
+extern void config_client_stop(ConfigClient *client);
 
 /* 全局客户端实例，用于信号处理 */
 static ConfigClient *g_demo_client = NULL;
+static int g_running = 1;
 
 /**
  * 配置变更回调示例
@@ -32,12 +54,10 @@ static ConfigClient *g_demo_client = NULL;
 void on_config_change(const char *key, const char *value) {
     printf("\n[Callback] Config changed: %s = %s\n", key, value);
     
-    /* 示例：根据配置键执行不同操作 */
     if (strcmp(key, "log.level") == 0) {
         printf("  -> Log level changed to: %s\n", value);
     } else if (strcmp(key, "database.host") == 0) {
         printf("  -> Database host changed to: %s\n", value);
-        /* 实际应用中可在此重新初始化数据库连接 */
     } else if (strcmp(key, "database.port") == 0) {
         printf("  -> Database port changed to: %s\n", value);
     } else if (strcmp(key, "feature.enabled") == 0) {
@@ -50,6 +70,7 @@ void on_config_change(const char *key, const char *value) {
  */
 static void signal_handler(int sig) {
     printf("\n[Signal] Received signal %d, shutting down...\n", sig);
+    g_running = 0;
     if (g_demo_client) {
         g_demo_client->running = 0;
     }
@@ -64,9 +85,6 @@ int main() {
     signal(SIGINT, signal_handler);
     signal(SIGTERM, signal_handler);
     
-    /* 初始化 CURL */
-    curl_global_init(CURL_GLOBAL_DEFAULT);
-    
     /* 配置客户端 */
     Config config = {
         .baseUrl = "http://localhost:8080",
@@ -92,7 +110,7 @@ int main() {
     printf("\n[Demo] Running... Press Ctrl+C to stop\n\n");
     
     int iterations = 0;
-    while (client.running && iterations < 10) {
+    while (g_running && iterations < 10) {
         sleep(5);
         
         /* 演示：定期读取并打印配置 */
@@ -106,7 +124,7 @@ int main() {
         printf("  database.port    = %s\n", dbPort ? dbPort : "(not set)");
         printf("  log.level        = %s\n", logLevel ? logLevel : "(not set)");
         printf("  feature.enabled  = %s\n", featureEnabled ? featureEnabled : "(not set)");
-        printf("  Total configs: %d\n\n", client.configs.count);
+        printf("\n");
         
         iterations++;
     }