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/getbuildinfo.c

@@ -0,0 +1,67 @@
+#include "Python.h"
+
+#ifndef DONT_HAVE_STDIO_H
+#include <stdio.h>
+#endif
+
+#ifndef DATE
+#ifdef __DATE__
+#define DATE __DATE__
+#else
+#define DATE "xx/xx/xx"
+#endif
+#endif
+
+#ifndef TIME
+#ifdef __TIME__
+#define TIME __TIME__
+#else
+#define TIME "xx:xx:xx"
+#endif
+#endif
+
+/* XXX Only unix build process has been tested */
+#ifndef GITVERSION
+#define GITVERSION ""
+#endif
+#ifndef GITTAG
+#define GITTAG ""
+#endif
+#ifndef GITBRANCH
+#define GITBRANCH ""
+#endif
+
+const char *
+Py_GetBuildInfo(void)
+{
+    static char buildinfo[50 + sizeof(GITVERSION) +
+                          ((sizeof(GITTAG) > sizeof(GITBRANCH)) ?
+                           sizeof(GITTAG) : sizeof(GITBRANCH))];
+    const char *revision = _Py_gitversion();
+    const char *sep = *revision ? ":" : "";
+    const char *gitid = _Py_gitidentifier();
+    if (!(*gitid))
+        gitid = "default";
+    PyOS_snprintf(buildinfo, sizeof(buildinfo),
+                  "%s%s%s, %.20s, %.9s", gitid, sep, revision,
+                  DATE, TIME);
+    return buildinfo;
+}
+
+const char *
+_Py_gitversion(void)
+{
+    return GITVERSION;
+}
+
+const char *
+_Py_gitidentifier(void)
+{
+    const char *gittag, *gitid;
+    gittag = GITTAG;
+    if ((*gittag) && strcmp(gittag, "undefined") != 0)
+        gitid = gittag;
+    else
+        gitid = GITBRANCH;
+    return gitid;
+}