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/cpython-2.7.18/Modules/config.c.in

@@ -0,0 +1,65 @@
+/* -*- C -*- ***********************************************
+Copyright (c) 2000, BeOpen.com.
+Copyright (c) 1995-2000, Corporation for National Research Initiatives.
+Copyright (c) 1990-1995, Stichting Mathematisch Centrum.
+All rights reserved.
+
+See the file "Misc/COPYRIGHT" for information on usage and
+redistribution of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+******************************************************************/
+
+/* Module configuration */
+
+/* !!! !!! !!! This file is edited by the makesetup script !!! !!! !!! */
+
+/* This file contains the table of built-in modules.
+   See init_builtin() in import.c. */
+
+#include "Python.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* -- ADDMODULE MARKER 1 -- */
+
+extern void PyMarshal_Init(void);
+extern void initimp(void);
+extern void initgc(void);
+extern void init_ast(void);
+extern void _PyWarnings_Init(void);
+
+struct _inittab _PyImport_Inittab[] = {
+
+/* -- ADDMODULE MARKER 2 -- */
+
+    /* This module lives in marshal.c */
+    {"marshal", PyMarshal_Init},
+
+    /* This lives in import.c */
+    {"imp", initimp},
+
+    /* This lives in Python/Python-ast.c */
+    {"_ast", init_ast},
+
+    /* These entries are here for sys.builtin_module_names */
+    {"__main__", NULL},
+    {"__builtin__", NULL},
+    {"sys", NULL},
+    {"exceptions", NULL},
+
+    /* This lives in gcmodule.c */
+    {"gc", initgc},
+
+    /* This lives in _warnings.c */
+    {"_warnings", _PyWarnings_Init},
+
+    /* Sentinel */
+    {0, 0}
+};
+
+
+#ifdef __cplusplus
+}
+#endif