docs: Fix core module documentation discrepancies

Fixed the following issues in XCEngine Core module documentation:
- Added 'using namespace XCEngine::Core;' to all code examples that use
  Core types (Event, FileWriter, etc.) without full namespace qualification
- Added missing '#include <XCEngine/Containers/String.h>' to FileWriter
  examples that use Containers::String
- Added '#include <string>' to Flush.md example using std::to_string

Affected files:
- core/core.md: Added using directive and Containers include
- event/*.md: Added using namespace to all 8 event doc files
- filewriter/*.md: Added using namespace and proper includes to all 6 files

docs/api/core/filewriter/Close.md

@@ -17,6 +17,8 @@ void Close();
 ```cpp
 #include <XCEngine/Core/FileWriter.h>
 
+using namespace XCEngine::Core;
+
 FileWriter writer;
 
 if (writer.Open("data.txt")) {

docs/api/core/filewriter/Flush.md

@@ -18,6 +18,9 @@ bool Flush();
 
 ```cpp
 #include <XCEngine/Core/FileWriter.h>
+#include <string>
+
+using namespace XCEngine::Core;
 
 FileWriter writer("output.txt");
 if (writer.IsOpen()) {

docs/api/core/filewriter/IsOpen.md

@@ -19,6 +19,8 @@ bool IsOpen() const;
 ```cpp
 #include <XCEngine/Core/FileWriter.h>
 
+using namespace XCEngine::Core;
+
 FileWriter writer1;
 if (writer1.IsOpen()) {
     // 不会执行

docs/api/core/filewriter/Open.md

@@ -23,6 +23,8 @@ bool Open(const char* filePath, bool append = false);
 ```cpp
 #include <XCEngine/Core/FileWriter.h>
 
+using namespace XCEngine::Core;
+
 FileWriter writer;
 
 // 打开文件（覆盖模式）

docs/api/core/filewriter/Write.md

@@ -26,6 +26,8 @@ bool Write(const Containers::String& str);
 #include <XCEngine/Core/FileWriter.h>
 #include <XCEngine/Containers/String.h>
 
+using namespace XCEngine::Core;
+
 FileWriter writer("output.txt");
 if (writer.IsOpen()) {
     // 写入原始字符数组

docs/api/core/filewriter/filewriter.md

@@ -51,6 +51,9 @@
 
 ```cpp
 #include <XCEngine/Core/FileWriter.h>
+#include <XCEngine/Containers/String.h>
+
+using namespace XCEngine::Core;
 
 // 方式1：构造时打开
 FileWriter writer("output.txt", false);