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/event/Clear.md

@@ -17,6 +17,8 @@ void Clear();
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> event;
 
 // 订阅多个回调

docs/api/core/event/Invoke.md

@@ -20,6 +20,8 @@ void Invoke(Args... args);
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 // 无参数事件
 Event<> frameEndEvent;
 frameEndEvent.Subscribe([]() {

docs/api/core/event/ProcessUnsubscribes.md

@@ -17,6 +17,8 @@ void ProcessUnsubscribes();
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> event;
 
 // 订阅

docs/api/core/event/Subscribe.md

@@ -22,6 +22,8 @@ uint64_t Subscribe(Callback callback);
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 // 定义事件
 Event<int, float> damageEvent;
 

docs/api/core/event/Unsubscribe.md

@@ -20,6 +20,8 @@ void Unsubscribe(uint64_t id);
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> someEvent;
 
 // 订阅

docs/api/core/event/begin.md

@@ -19,6 +19,8 @@ Iterator begin();
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> event;
 event.Subscribe([](int v) { printf("Callback 1: %d\n", v); });
 event.Subscribe([](int v) { printf("Callback 2: %d\n", v); });

docs/api/core/event/end.md

@@ -19,6 +19,8 @@ Iterator end();
 ```cpp
 #include <XCEngine/Core/Event.h>
 
+using namespace XCEngine::Core;
+
 Event<int> event;
 event.Subscribe([](int v) { printf("Callback: %d\n", v); });
 

docs/api/core/event/event.md

@@ -63,6 +63,10 @@
 ## 使用示例
 
 ```cpp
+#include <XCEngine/Core/Event.h>
+
+using namespace XCEngine::Core;
+
 // 定义事件（无参数）
 Event<> frameStartEvent;