port(2P step 3c): stage python27.zip into the mobile sandbox

CPython opens the stdlib zip with its own stdio, so it needs a real path.
On the desktop res:// is a directory and globalize_path() is enough; in an
exported build res:// is inside the PCK, which nothing outside Godot reads.

  make_stdlib_zip.py now also writes <out>.sha256
  mtpython_stdlib_project copies python27.zip + .sha256 into project/
    (gitignored), and both export presets' include_filter lists them, so
    they enter the PCK — checked with --export-pack
  extension/src/python_stdlib.{h,cpp} (Metin2Python.stdlib_path in
    GDScript): reads the bytes out of the PCK, writes
    user://python27.zip.part, hashes the file as written against the
    shipped digest, then renames. A sandbox copy is reused only when it
    matches, so a first start killed mid-copy and a zip replaced by a new
    build both re-stage instead of feeding zipimport a torn file.
  PythonHost::SetDefaultStdLibPath / DefaultStdLibPath() answer with that
    path ($MT_PYTHON_STDLIB still wins). python_stdlib.cpp is the only unit
    that knows res:// / user://; port_platform stays godot-free.

Also fixes the Windows gate, which step 3b broke: without mtpython the
MinGW build compiled UserInterface/StdAfx.h (it includes ScriptLib/StdAfx.h,
as the original PCH does). port/CMakeLists.txt excludes that header and
PythonPackModule.cpp with ScriptLib, and platform/CMakeLists.txt excludes
platform/ScriptLib/ the same way.

Not done: on-device staging. The macOS test drives the mobile path with
force_stage=true, and nothing in the app boot calls it yet — the
interpreter only starts in the process with 2V0.

gates: python_stdlib_test.gd PASS (in-place path, staging, sha256, bytes
equal res://, no .part, ZIPReader finds encodings/__init__.py, no re-copy
on a second call, corrupted copy re-staged) · macOS ctest 27/27 ·
mingw + android + ios port_platform compile clean · port_map.py check 0
errors · key leak check 8/8 none.

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

extension/CMakeLists.txt

@@ -198,10 +198,29 @@ add_library(mtgodot ${MT_LIB_KIND}
     src/net/m2_client.cpp
     src/proto/proto_node.cpp
     src/pack40250_node.cpp
+    src/python_stdlib.cpp
 )
 target_compile_features(mtgodot PRIVATE cxx_std_20)
 target_link_libraries(mtgodot PRIVATE godot::cpp xrender::libgr2 xrender::formats mtnet mtproto port_platform)
 
+# The embedded interpreter's standard library (批次 2P step 3c). python27.zip and its digest are
+# copied next to project.godot so the exporter packs them into the APK/IPA (the export presets'
+# include_filter lists them); src/python_stdlib.cpp stages them out of the PCK at runtime.
+if(TARGET mtpython_stdlib)
+    target_compile_definitions(mtgodot PRIVATE MTGODOT_HAVE_PYTHON)
+    # A target rather than a custom command on the two files: the command that produces them lives in
+    # third_party/, and a Makefile generator only resolves a generated-file dependency inside the
+    # directory that declared it. copy_if_different keeps the no-op build a compare.
+    add_custom_target(mtpython_stdlib_project ALL
+        COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                "${MT_PYTHON_STDLIB_ZIP}" "${CMAKE_SOURCE_DIR}/project/python27.zip"
+        COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                "${MT_PYTHON_STDLIB_SHA}" "${CMAKE_SOURCE_DIR}/project/python27.zip.sha256"
+        COMMENT "Copying python27.zip into project/ (res://)")
+    add_dependencies(mtpython_stdlib_project mtpython_stdlib)
+    add_dependencies(mtgodot mtpython_stdlib_project)
+endif()
+
 # Godot's .gdextension expects, per platform:
 #   project/bin/libmtgodot.macos.template_{debug,release}.dylib
 #   project/bin/libmtgodot.ios.template_{debug,release}.a