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/cgensupport.h

@@ -0,0 +1,64 @@
+#ifndef Py_CGENSUPPORT_H
+#define Py_CGENSUPPORT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Definitions used by cgen output */
+
+/* XXX This file is obsolete.  It is *only* used by glmodule.c. */
+
+typedef char *string;
+
+#define mknewlongobject(x) PyInt_FromLong(x)
+#define mknewshortobject(x) PyInt_FromLong((long)x)
+#define mknewfloatobject(x) PyFloat_FromDouble(x)
+#define mknewcharobject(ch) Py_BuildValue("c", ch)
+
+#define getichararg PyArg_GetChar
+#define getidoublearray PyArg_GetDoubleArray
+#define getifloatarg PyArg_GetFloat
+#define getifloatarray PyArg_GetFloatArray
+#define getilongarg PyArg_GetLong
+#define getilongarray PyArg_GetLongArray
+#define getilongarraysize PyArg_GetLongArraySize
+#define getiobjectarg PyArg_GetObject
+#define getishortarg PyArg_GetShort
+#define getishortarray PyArg_GetShortArray
+#define getishortarraysize PyArg_GetShortArraySize
+#define getistringarg PyArg_GetString
+
+extern int PyArg_GetObject(PyObject *args, int nargs,
+                           int i, PyObject **p_a);
+extern int PyArg_GetLong(PyObject *args, int nargs,
+                         int i, long *p_a);
+extern int PyArg_GetShort(PyObject *args, int nargs,
+                          int i, short *p_a);
+extern int PyArg_GetFloat(PyObject *args, int nargs,
+                          int i, float *p_a);
+extern int PyArg_GetString(PyObject *args, int nargs,
+                           int i, string *p_a);
+extern int PyArg_GetChar(PyObject *args, int nargs,
+                         int i, char *p_a);
+extern int PyArg_GetLongArray(PyObject *args, int nargs,
+                            int i, int n, long *p_a);
+extern int PyArg_GetShortArray(PyObject *args, int nargs,
+                            int i, int n, short *p_a);
+extern int PyArg_GetDoubleArray(PyObject *args, int nargs,
+                                int i, int n, double *p_a);
+extern int PyArg_GetFloatArray(PyObject *args, int nargs,
+                               int i, int n, float *p_a);
+extern int PyArg_GetLongArraySize(PyObject *args, int nargs,
+                                  int i, long *p_a);
+extern int PyArg_GetShortArraySize(PyObject *args, int nargs,
+                                int i, short *p_a);
+extern int PyArg_GetDoubleArraySize(PyObject *args, int nargs,
+                                    int i, double *p_a);
+extern int PyArg_GetFloatArraySize(PyObject *args, int nargs,
+                                   int i, float *p_a);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_CGENSUPPORT_H */