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/Python/importdl.h

@@ -0,0 +1,53 @@
+#ifndef Py_IMPORTDL_H
+#define Py_IMPORTDL_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Definitions for dynamic loading of extension modules */
+enum filetype {
+    SEARCH_ERROR,
+    PY_SOURCE,
+    PY_COMPILED,
+    C_EXTENSION,
+    PY_RESOURCE, /* Mac only */
+    PKG_DIRECTORY,
+    C_BUILTIN,
+    PY_FROZEN,
+    PY_CODERESOURCE, /* Mac only */
+    IMP_HOOK
+};
+
+struct filedescr {
+    char *suffix;
+    char *mode;
+    enum filetype type;
+};
+extern struct filedescr * _PyImport_Filetab;
+extern const struct filedescr _PyImport_DynLoadFiletab[];
+
+extern PyObject *_PyImport_LoadDynamicModule(char *name, char *pathname,
+                                             FILE *);
+
+/* Max length of module suffix searched for -- accommodates "module.slb" */
+#define MAXSUFFIXSIZE 12
+
+#ifdef MS_WINDOWS
+#include <windows.h>
+typedef FARPROC dl_funcptr;
+#else
+#if defined(PYOS_OS2) && !defined(PYCC_GCC)
+#include <os2def.h>
+typedef int (* APIENTRY dl_funcptr)();
+#else
+typedef void (*dl_funcptr)(void);
+#endif
+#endif
+
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_IMPORTDL_H */