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/cryptmodule.c

@@ -0,0 +1,49 @@
+/* cryptmodule.c - by Steve Majewski
+ */
+
+#include "Python.h"
+
+#include <sys/types.h>
+
+#ifdef __VMS
+#include <openssl/des.h>
+#endif
+
+/* Module crypt */
+
+
+static PyObject *crypt_crypt(PyObject *self, PyObject *args)
+{
+    char *word, *salt;
+#ifndef __VMS
+    extern char * crypt(const char *, const char *);
+#endif
+
+    if (!PyArg_ParseTuple(args, "ss:crypt", &word, &salt)) {
+        return NULL;
+    }
+    /* On some platforms (AtheOS) crypt returns NULL for an invalid
+       salt. Return None in that case. XXX Maybe raise an exception?  */
+    return Py_BuildValue("s", crypt(word, salt));
+
+}
+
+PyDoc_STRVAR(crypt_crypt__doc__,
+"crypt(word, salt) -> string\n\
+word will usually be a user's password. salt is a 2-character string\n\
+which will be used to select one of 4096 variations of DES. The characters\n\
+in salt must be either \".\", \"/\", or an alphanumeric character. Returns\n\
+the hashed password as a string, which will be composed of characters from\n\
+the same alphabet as the salt.");
+
+
+static PyMethodDef crypt_methods[] = {
+    {"crypt",           crypt_crypt, METH_VARARGS, crypt_crypt__doc__},
+    {NULL,              NULL}           /* sentinel */
+};
+
+PyMODINIT_FUNC
+initcrypt(void)
+{
+    Py_InitModule("crypt", crypt_methods);
+}