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/Misc/README.OpenBSD

@@ -0,0 +1,38 @@
+
+2005-01-08
+
+If you are have a problem building on OpenBSD and see output like this
+while running configure:
+
+checking curses.h presence... yes
+configure: WARNING: curses.h: present but cannot be compiled
+configure: WARNING: curses.h: check for missing prerequisite headers?
+configure: WARNING: curses.h: see the Autoconf documentation
+configure: WARNING: curses.h: section "Present But Cannot Be Compiled"
+configure: WARNING: curses.h: proceeding with the preprocessor's result
+configure: WARNING: curses.h: in the future, the compiler will take precedence
+
+there is likely a problem that will prevent building python.  
+If you see the messages above and are able to completely build python,
+please tell python-dev@python.org indicating your version of OpenBSD
+and any other relevant system configuration.
+
+The build error that occurs while making may look something like this:
+
+    /usr/include/sys/event.h:53: error: syntax error before "u_int"
+    /usr/include/sys/event.h:55: error: syntax error before "u_short"
+
+To fix this problem, you will probably need update Python's configure
+script to disable certain options.  Search for a line that looks like:
+
+    OpenBSD/2.* | OpenBSD/3.@<:@012345678@:>@)
+
+If your version is not in that list, e.g., 3.9, add the version
+number.  In this case, you would just need to add a 9 after the 8.
+If you modify configure.ac, you will need to regenerate configure
+with autoconf.
+
+If your version is already in the list, this is not a known problem.
+Please submit a bug report here:
+
+    http://sourceforge.net/tracker/?group_id=5470&atid=105470