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/asset_io.h

@@ -7,6 +7,9 @@
 //   - user:// (extracted cache)
 //   - absolute OS paths (dev: AssetRoot points at mtgodot-poc/assets/)
 // godot::FileAccess handles all three transparently.
+//   - pack://<40250 path> (e.g. pack://locale/en/item_proto, pack://d:/ymir work/pc/...): the 40250 packs,
+//     read through the ported CEterPackManager once Metin2Pack.initialize(<Client dir>) has run
+//     (platform/PackBackend.h). The path after pack:// is passed as UTF-8.
 #pragma once
 
 #include <godot_cpp/variant/packed_byte_array.hpp>
@@ -20,6 +23,8 @@
 
 namespace mtgodot {
 
+inline constexpr const char *PACK_SCHEME = "pack://";
+
 // Whole-file read. Empty PackedByteArray on failure (path missing / unreadable).
 godot::PackedByteArray read_file(const godot::String &path);
 bool file_exists(const godot::String &path); // FileAccess::file_exists wrapper