fix: Windows native code bugs and dedup

- fix unsigned underflow in Utf8FromUtf16 when WideCharToMultiByte fails
- fix inverted freopen_s logic in console attach
- remove misleading WS_EX_APPWINDOW from GWL_STYLE mask
- close mutex handle on normal exit
- extract SetHDRStateForTarget, EnableComposition/DisableComposition, DisplayModeToMap helpers
- cache GetWindowsVersion result

windows/runner/main.cpp

@@ -57,5 +57,6 @@ int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
   }
 
   ::CoUninitialize();
+  CloseHandle(mutex);
   return EXIT_SUCCESS;
 }