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/pack40250_node.cpp

@@ -0,0 +1,35 @@
+#include "pack40250_node.h"
+
+#include <godot_cpp/core/class_db.hpp>
+
+#include "asset_io.h"
+#include "platform/PackBackend.h"
+
+using namespace godot;
+
+namespace mtgodot {
+
+bool Metin2Pack::initialize(const String &client_dir) {
+	return mtpack40250::initialize(client_dir.utf8().get_data());
+}
+
+bool Metin2Pack::is_ready() {
+	return mtpack40250::ready();
+}
+
+bool Metin2Pack::exists(const String &name) {
+	return file_exists(String(PACK_SCHEME) + name);
+}
+
+PackedByteArray Metin2Pack::get_bytes(const String &name) {
+	return read_file(String(PACK_SCHEME) + name);
+}
+
+void Metin2Pack::_bind_methods() {
+	ClassDB::bind_static_method("Metin2Pack", D_METHOD("initialize", "client_dir"), &Metin2Pack::initialize);
+	ClassDB::bind_static_method("Metin2Pack", D_METHOD("is_ready"), &Metin2Pack::is_ready);
+	ClassDB::bind_static_method("Metin2Pack", D_METHOD("exists", "name"), &Metin2Pack::exists);
+	ClassDB::bind_static_method("Metin2Pack", D_METHOD("get_bytes", "name"), &Metin2Pack::get_bytes);
+}
+
+} // namespace mtgodot