port 2P step 1: vendor CPython 2.7.18 as the mtpython static library

The embedded 40250 script layer needs its own interpreter on five platforms.
2.7 is end-of-life, so nothing can be fetched from the target SDKs and the
source is vendored (official 2.7.18 tarball, trimmed to 25MB; Lib/ stays for
step 3's python27.zip).

cpython-2.7.18/CMakeLists.txt builds one `mtpython` static library from exactly
the 133 objects the reference libpython2.7.a contains, off by default behind
-DMTGODOT_EMBED_PYTHON=ON. The source list and the built-in module table
(config/config.c, 39 entries) are shared; pyconfig.h is a probe result and is
not, so each platform keeps its own under config/<platform>/, regenerated by
tools/py_embed/gen_pyconfig.sh and checked against the shared table.

Three vendor patches, documented in docs/THIRD-PARTY.md: configure/configure.ac
learn arm64 on macOS, and posixmodule.c undefines the process-control calls it
hard-defines past pyconfig.h when building for iOS.

macOS (ctest python.embed: every builtin imports, codecs and pickle work off the
vendored Lib/), Android arm64 and iOS arm64 build. Linux needs a Linux host and
Windows needs a MinGW-vs-MSVC decision; both are noted for step 4.

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

extension/third_party/CMakeLists.txt

@@ -44,3 +44,10 @@ add_library(minilzo STATIC minilzo/minilzo.c)
 target_include_directories(minilzo PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/minilzo")
 set_target_properties(minilzo PROPERTIES C_STANDARD 99)
 add_library(mt3p::minilzo ALIAS minilzo)
+
+# --- CPython 2.7.18 (embedded 40250 script layer, 批次 2P) ----------------------
+# Off by default: it only needs to build where the script layer is actually linked or tested.
+option(MTGODOT_EMBED_PYTHON "Build the embedded CPython 2.7.18 static library" OFF)
+if(MTGODOT_EMBED_PYTHON)
+    add_subdirectory(cpython-2.7.18 EXCLUDE_FROM_ALL)
+endif()