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/errors.py

@@ -0,0 +1,57 @@
+# Copyright (C) 2001-2006 Python Software Foundation
+# Author: Barry Warsaw
+# Contact: email-sig@python.org
+
+"""email package exception classes."""
+
+
+
+class MessageError(Exception):
+    """Base class for errors in the email package."""
+
+
+class MessageParseError(MessageError):
+    """Base class for message parsing errors."""
+
+
+class HeaderParseError(MessageParseError):
+    """Error while parsing headers."""
+
+
+class BoundaryError(MessageParseError):
+    """Couldn't find terminating boundary."""
+
+
+class MultipartConversionError(MessageError, TypeError):
+    """Conversion to a multipart is prohibited."""
+
+
+class CharsetError(MessageError):
+    """An illegal charset was given."""
+
+
+
+# These are parsing defects which the parser was able to work around.
+class MessageDefect:
+    """Base class for a message defect."""
+
+    def __init__(self, line=None):
+        self.line = line
+
+class NoBoundaryInMultipartDefect(MessageDefect):
+    """A message claimed to be a multipart but had no boundary parameter."""
+
+class StartBoundaryNotFoundDefect(MessageDefect):
+    """The claimed start boundary was never found."""
+
+class FirstHeaderLineIsContinuationDefect(MessageDefect):
+    """A message had a continuation line as its first header line."""
+
+class MisplacedEnvelopeHeaderDefect(MessageDefect):
+    """A 'Unix-from' header was found in the middle of a header block."""
+
+class MalformedHeaderDefect(MessageDefect):
+    """Found a header that was missing a colon, or was otherwise malformed."""
+
+class MultipartInvariantViolationDefect(MessageDefect):
+    """A message claimed to be a multipart but no subparts were found."""