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/core.md

@@ -71,10 +71,13 @@ UniqueRef<T> MakeUnique(Args&&... args);
 
 ```cpp
 #include <XCEngine/Core/Core.h>
+#include <XCEngine/Containers/String.h>
+
+using namespace XCEngine::Core;
 
 // 使用类型别名
-Core::uint32 value = 100;
-Core::byte data[4];
+uint32 value = 100;
+byte data[4];
 
 // 使用智能指针
 auto ref = MakeRef<MyClass>();