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.h

@@ -0,0 +1,28 @@
+#pragma once
+// Metin2Pack — GDScript access to the 40250 packs (platform/PackBackend.h).
+//
+//   Metin2Pack.initialize(AssetRoot.client_path())   # registers <Client>/pack/Index once
+//   var b := Metin2Pack.get_bytes("locale/en/item_proto")
+//
+// Every native reader that takes a path accepts "pack://<40250 path>" through asset_io.
+
+#include <godot_cpp/classes/object.hpp>
+#include <godot_cpp/variant/packed_byte_array.hpp>
+#include <godot_cpp/variant/string.hpp>
+
+namespace mtgodot {
+
+class Metin2Pack : public godot::Object {
+	GDCLASS(Metin2Pack, godot::Object)
+
+public:
+	static bool initialize(const godot::String &client_dir);
+	static bool is_ready();
+	static bool exists(const godot::String &name);
+	static godot::PackedByteArray get_bytes(const godot::String &name);
+
+protected:
+	static void _bind_methods();
+};
+
+} // namespace mtgodot