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/Include/bytes_methods.h

@@ -0,0 +1,75 @@
+#ifndef Py_BYTES_CTYPE_H
+#define Py_BYTES_CTYPE_H
+
+/*
+ * The internal implementation behind PyString (bytes) and PyBytes (buffer)
+ * methods of the given names, they operate on ASCII byte strings.
+ */
+extern PyObject* _Py_bytes_isspace(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_isalpha(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_isalnum(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_isdigit(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_islower(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_isupper(const char *cptr, Py_ssize_t len);
+extern PyObject* _Py_bytes_istitle(const char *cptr, Py_ssize_t len);
+
+/* These store their len sized answer in the given preallocated *result arg. */
+extern void _Py_bytes_lower(char *result, const char *cptr, Py_ssize_t len);
+extern void _Py_bytes_upper(char *result, const char *cptr, Py_ssize_t len);
+extern void _Py_bytes_title(char *result, char *s, Py_ssize_t len);
+extern void _Py_bytes_capitalize(char *result, char *s, Py_ssize_t len);
+extern void _Py_bytes_swapcase(char *result, char *s, Py_ssize_t len);
+
+/* Shared __doc__ strings. */
+extern const char _Py_isspace__doc__[];
+extern const char _Py_isalpha__doc__[];
+extern const char _Py_isalnum__doc__[];
+extern const char _Py_isdigit__doc__[];
+extern const char _Py_islower__doc__[];
+extern const char _Py_isupper__doc__[];
+extern const char _Py_istitle__doc__[];
+extern const char _Py_lower__doc__[];
+extern const char _Py_upper__doc__[];
+extern const char _Py_title__doc__[];
+extern const char _Py_capitalize__doc__[];
+extern const char _Py_swapcase__doc__[];
+
+/* These are left in for backward compatibility and will be removed
+   in 2.8/3.2 */
+#define ISLOWER(c)  Py_ISLOWER(c)
+#define ISUPPER(c)  Py_ISUPPER(c)
+#define ISALPHA(c)  Py_ISALPHA(c)
+#define ISDIGIT(c)  Py_ISDIGIT(c)
+#define ISXDIGIT(c) Py_ISXDIGIT(c)
+#define ISALNUM(c)  Py_ISALNUM(c)
+#define ISSPACE(c)  Py_ISSPACE(c)
+
+#undef islower
+#define islower(c) undefined_islower(c)
+#undef isupper
+#define isupper(c) undefined_isupper(c)
+#undef isalpha
+#define isalpha(c) undefined_isalpha(c)
+#undef isdigit
+#define isdigit(c) undefined_isdigit(c)
+#undef isxdigit
+#define isxdigit(c) undefined_isxdigit(c)
+#undef isalnum
+#define isalnum(c) undefined_isalnum(c)
+#undef isspace
+#define isspace(c) undefined_isspace(c)
+
+/* These are left in for backward compatibility and will be removed
+   in 2.8/3.2 */
+#define TOLOWER(c) Py_TOLOWER(c)
+#define TOUPPER(c) Py_TOUPPER(c)
+
+#undef tolower
+#define tolower(c) undefined_tolower(c)
+#undef toupper
+#define toupper(c) undefined_toupper(c)
+
+/* this is needed because some docs are shared from the .o, not static */
+#define PyDoc_STRVAR_shared(name,str) const char name[] = PyDoc_STR(str)
+
+#endif /* !Py_BYTES_CTYPE_H */