chore: 保存笔记本本地未提交改动(源码+新增文件，排除构建缓存/大文件)

c_demo/main.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+int main() {
+    printf("Hello from C project!\n");
+    printf("Built with Makefile\n");
+    return 0;
+}