Fix CA::MetalLayer undeclared in C++ TUs via forward declaration

<QuartzCore/CAMetalLayer.h> only defines an Objective-C @interface, not
the C++ CA::MetalLayer class from Metal-cpp. BabylonNative.cpp is compiled
as a pure C++ translation unit, so including that header does not make the
CA:: namespace visible.

Replace the include with a C++ forward declaration:
  namespace CA { class MetalLayer; }

This is sufficient for the pointer type alias in the header.
The full definition is provided by the Metal-cpp headers that BabylonNative
itself brings in when the implementation files are compiled.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: CedricGuillemet

Modules/@babylonjs/react-native/shared/BabylonNative.h

@@ -4,7 +4,10 @@
 
 #if defined(__APPLE__)
 #include <MetalKit/MTKView.h>
-#include <QuartzCore/CAMetalLayer.h>
+// Forward-declare CA::MetalLayer so BabylonNative.h is usable from pure C++
+// translation units. The ObjC header <QuartzCore/CAMetalLayer.h> only defines
+// the @interface, not the C++ CA:: namespace class from Metal-cpp.
+namespace CA { class MetalLayer; }
 #elif defined(ANDROID)
 #include <android/native_window.h>
 #elif WINAPI_FAMILY == WINAPI_FAMILY_APP