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/Objects/stringlib/README.txt

@@ -0,0 +1,40 @@
+bits shared by the stringobject and unicodeobject implementations (and
+possibly other modules, in a not too distant future).
+
+the stuff in here is included into relevant places; see the individual
+source files for details.
+
+--------------------------------------------------------------------
+the following defines used by the different modules:
+
+STRINGLIB_CHAR
+
+    the type used to hold a character (char or Py_UNICODE)
+
+STRINGLIB_EMPTY
+
+    a PyObject representing the empty string, only to be used if
+    STRINGLIB_MUTABLE is 0
+
+Py_ssize_t STRINGLIB_LEN(PyObject*)
+
+    returns the length of the given string object (which must be of the
+    right type)
+
+PyObject* STRINGLIB_NEW(STRINGLIB_CHAR*, Py_ssize_t)
+
+    creates a new string object
+
+STRINGLIB_CHAR* STRINGLIB_STR(PyObject*)
+
+    returns the pointer to the character data for the given string
+    object (which must be of the right type)
+
+int STRINGLIB_CHECK_EXACT(PyObject *)
+
+    returns true if the object is an instance of our type, not a subclass
+
+STRINGLIB_MUTABLE
+
+    must be 0 or 1 to tell the cpp macros in stringlib code if the object
+    being operated on is mutable or not