fix(core): Replace deprecated rncore.h with FBReactNativeSpec.h

rncore.h is deprecated in RN 0.84 and triggers -Werror.
Replace with FBReactNativeSpec.h and FBReactNativeSpec_ModuleProvider.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonis

samples/react-native/android/app/src/main/jni/OnLoad.cpp

@@ -33,7 +33,7 @@
 #include <autolinking.h>
 #include <fbjni/fbjni.h>
 #include <react/renderer/componentregistry/ComponentDescriptorProviderRegistry.h>
-#include <rncore.h>
+#include <FBReactNativeSpec.h>
 
 #ifdef REACT_NATIVE_APP_CODEGEN_HEADER
 #    include REACT_NATIVE_APP_CODEGEN_HEADER
@@ -91,7 +91,7 @@ javaModuleProvider(const std::string &name, const JavaTurboModule::InitParams &p
     // if (module != nullptr) {
     //    return module;
     // }
-    // return rncore_ModuleProvider(name, params);
+    // return FBReactNativeSpec_ModuleProvider(name, params);
 
     // We link app local modules if available
 #ifdef REACT_NATIVE_APP_MODULE_PROVIDER
@@ -102,7 +102,7 @@ javaModuleProvider(const std::string &name, const JavaTurboModule::InitParams &p
 #endif
 
     // We first try to look up core modules
-    if (auto module = rncore_ModuleProvider(name, params)) {
+    if (auto module = FBReactNativeSpec_ModuleProvider(name, params)) {
         return module;
     }