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/asdl.c

@@ -0,0 +1,64 @@
+#include "Python.h"
+#include "asdl.h"
+
+asdl_seq *
+asdl_seq_new(int size, PyArena *arena)
+{
+    asdl_seq *seq = NULL;
+    size_t n = (size ? (sizeof(void *) * (size - 1)) : 0);
+
+    /* check size is sane */
+    if (size < 0 || size == INT_MIN ||
+        (size && ((size - 1) > (PY_SIZE_MAX / sizeof(void *))))) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+
+    /* check if size can be added safely */
+    if (n > PY_SIZE_MAX - sizeof(asdl_seq)) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+
+    n += sizeof(asdl_seq);
+
+    seq = (asdl_seq *)PyArena_Malloc(arena, n);
+    if (!seq) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    memset(seq, 0, n);
+    seq->size = size;
+    return seq;
+}
+
+asdl_int_seq *
+asdl_int_seq_new(int size, PyArena *arena)
+{
+    asdl_int_seq *seq = NULL;
+    size_t n = (size ? (sizeof(void *) * (size - 1)) : 0);
+
+    /* check size is sane */
+    if (size < 0 || size == INT_MIN ||
+        (size && ((size - 1) > (PY_SIZE_MAX / sizeof(void *))))) {
+            PyErr_NoMemory();
+            return NULL;
+    }
+
+    /* check if size can be added safely */
+    if (n > PY_SIZE_MAX - sizeof(asdl_seq)) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+
+    n += sizeof(asdl_seq);
+
+    seq = (asdl_int_seq *)PyArena_Malloc(arena, n);
+    if (!seq) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    memset(seq, 0, n);
+    seq->size = size;
+    return seq;
+}