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/Include/osdefs.h

@@ -0,0 +1,63 @@
+#ifndef Py_OSDEFS_H
+#define Py_OSDEFS_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Operating system dependencies */
+
+/* Mod by chrish: QNX has WATCOM, but isn't DOS */
+#if !defined(__QNX__)
+#if defined(MS_WINDOWS) || defined(__BORLANDC__) || defined(__WATCOMC__) || defined(__DJGPP__) || defined(PYOS_OS2)
+#if defined(PYOS_OS2) && defined(PYCC_GCC)
+#define MAXPATHLEN 260
+#define SEP '/'
+#define ALTSEP '\\'
+#else
+#define SEP '\\'
+#define ALTSEP '/'
+#define MAXPATHLEN 256
+#endif
+#define DELIM ';'
+#endif
+#endif
+
+#ifdef RISCOS
+#define SEP '.'
+#define MAXPATHLEN 256
+#define DELIM ','
+#endif
+
+
+/* Filename separator */
+#ifndef SEP
+#define SEP '/'
+#endif
+
+/* Max pathname length */
+#ifdef __hpux
+#include <sys/param.h>
+#include <limits.h>
+#ifndef PATH_MAX
+#define PATH_MAX MAXPATHLEN
+#endif
+#endif
+
+#ifndef MAXPATHLEN
+#if defined(PATH_MAX) && PATH_MAX > 1024
+#define MAXPATHLEN PATH_MAX
+#else
+#define MAXPATHLEN 1024
+#endif
+#endif
+
+/* Search path entry delimiter */
+#ifndef DELIM
+#define DELIM ':'
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_OSDEFS_H */