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/genobject.h

@@ -0,0 +1,40 @@
+
+/* Generator object interface */
+
+#ifndef Py_GENOBJECT_H
+#define Py_GENOBJECT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct _frame; /* Avoid including frameobject.h */
+
+typedef struct {
+	PyObject_HEAD
+	/* The gi_ prefix is intended to remind of generator-iterator. */
+
+	/* Note: gi_frame can be NULL if the generator is "finished" */
+	struct _frame *gi_frame;
+
+	/* True if generator is being executed. */
+	int gi_running;
+    
+	/* The code object backing the generator */
+	PyObject *gi_code;
+
+	/* List of weak reference. */
+	PyObject *gi_weakreflist;
+} PyGenObject;
+
+PyAPI_DATA(PyTypeObject) PyGen_Type;
+
+#define PyGen_Check(op) PyObject_TypeCheck(op, &PyGen_Type)
+#define PyGen_CheckExact(op) (Py_TYPE(op) == &PyGen_Type)
+
+PyAPI_FUNC(PyObject *) PyGen_New(struct _frame *);
+PyAPI_FUNC(int) PyGen_NeedsFinalizing(PyGenObject *);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_GENOBJECT_H */