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/unicodedefs.h

@@ -0,0 +1,37 @@
+#ifndef STRINGLIB_UNICODEDEFS_H
+#define STRINGLIB_UNICODEDEFS_H
+
+/* this is sort of a hack.  there's at least one place (formatting
+   floats) where some stringlib code takes a different path if it's
+   compiled as unicode. */
+#define STRINGLIB_IS_UNICODE     1
+
+#define STRINGLIB_OBJECT         PyUnicodeObject
+#define STRINGLIB_CHAR           Py_UNICODE
+#define STRINGLIB_TYPE_NAME      "unicode"
+#define STRINGLIB_PARSE_CODE     "U"
+#define STRINGLIB_EMPTY          unicode_empty
+#define STRINGLIB_ISSPACE        Py_UNICODE_ISSPACE
+#define STRINGLIB_ISLINEBREAK    BLOOM_LINEBREAK
+#define STRINGLIB_ISDECIMAL      Py_UNICODE_ISDECIMAL
+#define STRINGLIB_TODECIMAL      Py_UNICODE_TODECIMAL
+#define STRINGLIB_TOUPPER        Py_UNICODE_TOUPPER
+#define STRINGLIB_TOLOWER        Py_UNICODE_TOLOWER
+#define STRINGLIB_FILL           Py_UNICODE_FILL
+#define STRINGLIB_STR            PyUnicode_AS_UNICODE
+#define STRINGLIB_LEN            PyUnicode_GET_SIZE
+#define STRINGLIB_NEW            PyUnicode_FromUnicode
+#define STRINGLIB_RESIZE         PyUnicode_Resize
+#define STRINGLIB_CHECK          PyUnicode_Check
+#define STRINGLIB_CHECK_EXACT    PyUnicode_CheckExact
+#define STRINGLIB_GROUPING       _PyUnicode_InsertThousandsGrouping
+
+#if PY_VERSION_HEX < 0x03000000
+#define STRINGLIB_TOSTR          PyObject_Unicode
+#else
+#define STRINGLIB_TOSTR          PyObject_Str
+#endif
+
+#define STRINGLIB_WANT_CONTAINS_OBJ 1
+
+#endif /* !STRINGLIB_UNICODEDEFS_H */