port 2D step 5b: PackInitialize + pack:// asset_io backend over the ported EterPackManager

Platform UserInterface PackInitialize/PackSingletons (40250 bootstrap, on-disk dbname
resolution), EterBase/FileLoader.cpp mirror, mtpack40250 backend behind asset_io's
pack:// scheme and a Metin2Pack node. port_eterpack_test now registers through
PackInitialize. Nothing reads pack:// at runtime yet; port-map stays TODO.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/src/platform/PackBackend.cpp

@@ -0,0 +1,60 @@
+#include "EterBase/StdAfx.h"
+#include "EterBase/MappedFile.h"
+#include "EterPack/EterPackManager.h"
+#include "UserInterface/UserInterface.h"
+#include "PackBackend.h"
+
+#include <mutex>
+
+#if defined(_WIN32)
+#include <io.h> // _access; elsewhere the <windows.h> shim declares it
+#endif
+
+namespace mtpack40250 {
+
+namespace {
+std::mutex g_init_mutex;
+bool g_initialized = false;
+bool g_ready = false;
+}
+
+bool initialize(const std::string& client_dir)
+{
+	std::lock_guard<std::mutex> lock(g_init_mutex);
+	if (g_initialized)
+		return g_ready;
+	const std::string folder = client_dir + "/pack";
+	if (_access((folder + "/Index").c_str(), 0) != 0)
+		return false; // nothing registered; a later call with the right directory may still succeed
+	g_initialized = true;
+	PackSingletons();
+	g_ready = PackInitialize(folder.c_str());
+	return g_ready;
+}
+
+bool ready()
+{
+	std::lock_guard<std::mutex> lock(g_init_mutex);
+	return g_ready;
+}
+
+bool read(const std::string& name, std::vector<uint8_t>& out)
+{
+	out.clear();
+	if (!ready())
+		return false;
+	CMappedFile file;
+	LPCVOID data = nullptr;
+	if (!CEterPackManager::Instance().Get(file, name.c_str(), &data))
+		return false;
+	const BYTE* bytes = static_cast<const BYTE*>(data);
+	out.assign(bytes, bytes + file.Size());
+	return true;
+}
+
+bool exists(const std::string& name)
+{
+	return ready() && CEterPackManager::Instance().isExist(name.c_str());
+}
+
+} // namespace mtpack40250