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/BeOS-NOTES

@@ -0,0 +1,43 @@
+Python for BeOS R5
+
+In Python-2.1, the standard version of the new setup.py program
+will not build the full complement of modules on BeOS.  Instead,
+please replace it with the special BeOS version in Misc/BeOS-setup.py.
+
+To build,
+
+   1)  cp Misc/BeOS-setup.py setup.py
+   2)  ./configure --prefix=/boot/home/config
+   3)  make
+
+The modules will all build, except termios which assumes some flags
+we don't have.  Put a libreadline.a in /boot/home/config/lib to get
+a readline.so for your interactive editing convenience;  NB, not
+libreadline.so, you want to link a static readline library into the
+dynamically loaded Python module.
+
+Test:
+
+   make test
+
+   The BeOS is Not UNIX category:
+ - test_select crashed -- select.error : (-2147459072, 'Bad file descriptor')
+ - test_socket crashed -- exceptions.AttributeError : SOCK_RAW
+ - test_fcntl crashed -- exceptions.IOError: [Errno -2147483643] Invalid argument
+
+   This one is funny!  BeOS does support large files, and that's why
+       we get this error:  the file is too big for my filesystem!
+ - test_largefile crashed -- exceptions.IOError: [Errno -2147459065]
+       No space left on device
+
+ - test_pickle crashed.  This is apparently a serious problem, "complex"
+       number objects reconstructed from a pickle don't compare equal to
+       their ancestors.  But it happens on BeOS PPC only, not Intel.
+
+Install:
+
+   make install
+
+
+Maintainer: None (please volunteer if you would like to see this port continue
+to exist!)