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/Lib/email/encoders.py

@@ -0,0 +1,82 @@
+# Copyright (C) 2001-2006 Python Software Foundation
+# Author: Barry Warsaw
+# Contact: email-sig@python.org
+
+"""Encodings and related functions."""
+
+__all__ = [
+    'encode_7or8bit',
+    'encode_base64',
+    'encode_noop',
+    'encode_quopri',
+    ]
+
+import base64
+
+from quopri import encodestring as _encodestring
+
+
+
+def _qencode(s):
+    enc = _encodestring(s, quotetabs=True)
+    # Must encode spaces, which quopri.encodestring() doesn't do
+    return enc.replace(' ', '=20')
+
+
+def _bencode(s):
+    # We can't quite use base64.encodestring() since it tacks on a "courtesy
+    # newline".  Blech!
+    if not s:
+        return s
+    hasnewline = (s[-1] == '\n')
+    value = base64.encodestring(s)
+    if not hasnewline and value[-1] == '\n':
+        return value[:-1]
+    return value
+
+
+
+def encode_base64(msg):
+    """Encode the message's payload in Base64.
+
+    Also, add an appropriate Content-Transfer-Encoding header.
+    """
+    orig = msg.get_payload()
+    encdata = _bencode(orig)
+    msg.set_payload(encdata)
+    msg['Content-Transfer-Encoding'] = 'base64'
+
+
+
+def encode_quopri(msg):
+    """Encode the message's payload in quoted-printable.
+
+    Also, add an appropriate Content-Transfer-Encoding header.
+    """
+    orig = msg.get_payload()
+    encdata = _qencode(orig)
+    msg.set_payload(encdata)
+    msg['Content-Transfer-Encoding'] = 'quoted-printable'
+
+
+
+def encode_7or8bit(msg):
+    """Set the Content-Transfer-Encoding header to 7bit or 8bit."""
+    orig = msg.get_payload()
+    if orig is None:
+        # There's no payload.  For backwards compatibility we use 7bit
+        msg['Content-Transfer-Encoding'] = '7bit'
+        return
+    # We play a trick to make this go fast.  If encoding to ASCII succeeds, we
+    # know the data must be 7bit, otherwise treat it as 8bit.
+    try:
+        orig.encode('ascii')
+    except UnicodeError:
+        msg['Content-Transfer-Encoding'] = '8bit'
+    else:
+        msg['Content-Transfer-Encoding'] = '7bit'
+
+
+
+def encode_noop(msg):
+    """Do nothing."""