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.cpp

@@ -2,19 +2,51 @@
 
 #include <godot_cpp/classes/file_access.hpp>
 
+#include "platform/PackBackend.h"
+
+#include <cstring>
+#include <string>
+#include <vector>
+
 using namespace godot;
 
 namespace mtgodot {
 
+namespace {
+bool pack_name(const String &path, std::string &name) {
+	if (!path.begins_with(PACK_SCHEME)) {
+		return false;
+	}
+	name = path.substr(String(PACK_SCHEME).length()).utf8().get_data();
+	return true;
+}
+} // namespace
+
 PackedByteArray read_file(const String &path) {
 	if (path.is_empty()) {
 		return PackedByteArray();
 	}
+	std::string name;
+	if (pack_name(path, name)) {
+		std::vector<uint8_t> bytes;
+		PackedByteArray out;
+		if (mtpack40250::read(name, bytes)) {
+			out.resize((int64_t)bytes.size());
+			if (!bytes.empty()) {
+				std::memcpy(out.ptrw(), bytes.data(), bytes.size());
+			}
+		}
+		return out;
+	}
 	// FileAccess::get_file_as_bytes handles res:// / user:// / absolute OS paths.
 	return FileAccess::get_file_as_bytes(path);
 }
 
 bool file_exists(const String &path) {
+	std::string name;
+	if (pack_name(path, name)) {
+		return mtpack40250::exists(name);
+	}
 	return !path.is_empty() && FileAccess::file_exists(path);
 }