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/platform/ScriptLib/PythonBoot.h

@@ -0,0 +1,41 @@
+#pragma once
+// Starting the script layer inside a host process (docs/PORT-PLAN.md 批次 2P step 4).
+//
+// 40250 runs the interpreter on Main()'s stack (UserInterface/UserInterface.cpp:419-434): construct
+// CPythonLauncher, Create(), RunMainScript(), Clear(), all inside one WinMain. Godot owns the process
+// here and calls in from GDScript, so the launcher has to outlive the call that starts it — that split
+// (Start / RunMainScript / Stop) is the only difference from the original control flow.
+//
+// It lives under platform/ScriptLib rather than platform/UserInterface because it is only buildable
+// with the embedded interpreter, and that is the directory both port_logic and port_platform drop when
+// `mtpython` is absent (extension/src/port/CMakeLists.txt).
+//
+// godot-free, like everything else under platform/: extension/src/python_host_node.cpp is what GDScript
+// talks to.
+
+#include <string>
+
+namespace PythonBoot
+{
+// PythonHost::Configure() -> CPythonLauncher -> Create() -> PythonHost::InstallStdLib(stdlib_path).
+// An empty stdlib_path means PythonHost::DefaultStdLibPath(). Starting twice is a no-op that succeeds.
+// On failure *error is set and the interpreter is shut down again, so the next attempt starts clean.
+// The packs must already be registered (platform/PackBackend.h), as 40250 registers them in Main()
+// before the launcher exists.
+bool Start(const char* stdlib_path, std::string* error);
+bool IsRunning();
+
+// 40250: RunMainScript(CPythonLauncher&, const char*) — the module initializers, __DEBUG__,
+// __COMMAND_LINE__ and RunFile("system.py"). Only the initializers that are ported are called; the rest
+// arrive with the 2V slices, which is why system.py still stops at its first import.
+// False leaves *error set to what the launcher reported through LogBox (platform/EterBase/LogBox.h).
+bool RunMainScript(const char* lpCmdLine, std::string* error);
+
+// CPythonLauncher::RunLine, with the reported text handed back rather than only shown. The host's own
+// check uses it to prove the interpreter and the standard library work in-process on a device that has
+// no 40250 packs, which is where RunMainScript cannot run at all.
+bool RunLine(const char* source, std::string* error);
+
+// 40250 Main() calls Clear() and lets the launcher leave scope.
+void Stop();
+}