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/xml/__init__.py

@@ -0,0 +1,41 @@
+"""Core XML support for Python.
+
+This package contains four sub-packages:
+
+dom -- The W3C Document Object Model.  This supports DOM Level 1 +
+       Namespaces.
+
+parsers -- Python wrappers for XML parsers (currently only supports Expat).
+
+sax -- The Simple API for XML, developed by XML-Dev, led by David
+       Megginson and ported to Python by Lars Marius Garshol.  This
+       supports the SAX 2 API.
+
+etree -- The ElementTree XML library.  This is a subset of the full
+       ElementTree XML release.
+
+"""
+
+
+__all__ = ["dom", "parsers", "sax", "etree"]
+
+_MINIMUM_XMLPLUS_VERSION = (0, 8, 4)
+
+
+try:
+    import _xmlplus
+except ImportError:
+    pass
+else:
+    try:
+        v = _xmlplus.version_info
+    except AttributeError:
+        # _xmlplus is too old; ignore it
+        pass
+    else:
+        if v >= _MINIMUM_XMLPLUS_VERSION:
+            import sys
+            _xmlplus.__path__.extend(__path__)
+            sys.modules[__name__] = _xmlplus
+        else:
+            del v