feat(python): 在 app 进程里起 40250 脚本层（2P step 4，macOS）

移植 UserInterface/RunMainScript 到 platform/ScriptLib/PythonBoot：40250 把
CPythonLauncher 放在 Main() 的栈上，这里进程归 Godot，launcher 必须活过创建它的
那次调用，所以拆成 Start / RunMainScript / RunLine / Stop。37 个模块初始化函数里
只调已移植的 initpack()，system.py 因此仍停在第一个 import。

脚本报错能传回 GDScript，靠的是 platform/EterBase/Debug.cpp 实现 LogBox/LogBoxf：
ScriptLib 的 Traceback() 在 PyErr_Fetch 之后只剩这一条路，LastLogBoxMessage()
（platform/EterBase/LogBox.h）是 traceback 唯一幸存的地方。

GDScript 侧新增 Metin2PythonHost（python_host_node.{h,cpp}）——唯一同时认识
godot-cpp 和 port 树的单元，PythonBoot 保持 godot-free。检查脚本
python_host_check.gd 分两段：解释器 + 标准库那段不需要 40250 的 pack，哪个平台都
能跑；system.py 那段需要 pack，没有就跳过。桌面入口 python_host_test.gd，真机入口
MT_TEST_MODE=python（导出模板不接受 --script）。

验证（macOS）：标准库可 import、codec 注册表可用、sys.path 非空，system.py 停在和
ctest port.python_launcher 完全相同的 ImportError: No module named app；
ctest 27/27。Android/iOS 进程内未做，正式启动路径仍不起解释器（2V0）。

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

extension/src/python_host_node.h

@@ -0,0 +1,38 @@
+#pragma once
+// Metin2PythonHost — the 40250 script layer inside the Godot process (docs/PORT-PLAN.md 批次 2P step 4).
+//
+//   Metin2Pack.initialize(AssetRoot.client_path())   # the packs system.py is read from
+//   if Metin2PythonHost.start() == "":
+//       var err := Metin2PythonHost.run_main_script("")
+//
+// Both calls answer with "" on success and with the reason otherwise, so GDScript never has to guess
+// from a bool. Until 批次 2V0 registers the C++ modules, run_main_script() is expected to come back with
+// system.py's first import error; that is what step 4 verifies on each target platform.
+//
+// This is the only unit that knows both godot-cpp and the port tree: the sequence itself lives in
+// platform/ScriptLib/PythonBoot.h, which stays godot-free.
+
+#include <godot_cpp/classes/object.hpp>
+#include <godot_cpp/variant/string.hpp>
+
+namespace mtgodot {
+
+class Metin2PythonHost : public godot::Object {
+	GDCLASS(Metin2PythonHost, godot::Object)
+
+public:
+	// Stages the standard library (Metin2Python::stdlib_path) when `stdlib_path` is empty, then starts
+	// the interpreter. "" on success, including when it was already running.
+	static godot::String start(const godot::String &stdlib_path);
+	static bool is_running();
+	// __DEBUG__, __COMMAND_LINE__ and system.py out of the packs. "" when system.py ran to the end.
+	static godot::String run_main_script(const godot::String &command_line);
+	// One line of Python in the same __main__ dictionary. "" when it ran.
+	static godot::String run_line(const godot::String &source);
+	static void stop();
+
+protected:
+	static void _bind_methods();
+};
+
+} // namespace mtgodot