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/PC/python_nt.rc

@@ -0,0 +1,74 @@
+// Resource script for Python core DLL.
+// Currently only holds version information.
+//
+#include "winver.h"
+
+#define MS_WINDOWS
+#include "modsupport.h"
+#include "patchlevel.h"
+#ifdef _DEBUG
+#   include "pythonnt_rc_d.h"
+#else
+#   include "pythonnt_rc.h"
+#endif
+
+/* e.g., 2.1a2
+ * PY_VERSION comes from patchevel.h
+ */
+#define PYTHON_VERSION PY_VERSION "\0"
+
+/* 64-bit version number as comma-separated list of 4 16-bit ints */
+#if PY_MICRO_VERSION > 64
+#   error "PY_MICRO_VERSION > 64"
+#endif
+#if PY_RELEASE_LEVEL > 99
+#   error "PY_RELEASE_LEVEL > 99"
+#endif
+#if PY_RELEASE_SERIAL > 9
+#   error "PY_RELEASE_SERIAL > 9"
+#endif
+#define PYVERSION64 PY_MAJOR_VERSION, PY_MINOR_VERSION, FIELD3, PYTHON_API_VERSION
+
+// String Tables
+STRINGTABLE DISCARDABLE
+BEGIN
+    1000,   MS_DLL_ID
+END
+
+/////////////////////////////////////////////////////////////////////////////
+//
+// Version
+//
+
+VS_VERSION_INFO VERSIONINFO
+ FILEVERSION PYVERSION64
+ PRODUCTVERSION PYVERSION64
+ FILEFLAGSMASK 0x3fL
+#ifdef _DEBUG
+ FILEFLAGS 0x1L
+#else
+ FILEFLAGS 0x0L
+#endif
+ FILEOS 0x40004L
+ FILETYPE 0x1L
+ FILESUBTYPE 0x0L
+BEGIN
+    BLOCK "StringFileInfo"
+    BEGIN
+        BLOCK "000004b0"
+        BEGIN
+            VALUE "CompanyName", "Python Software Foundation\0"
+            VALUE "FileDescription", "Python Core\0"
+            VALUE "FileVersion", PYTHON_VERSION
+            VALUE "InternalName", "Python DLL\0"
+            VALUE "LegalCopyright", "Copyright © 2001-2017 Python Software Foundation. Copyright © 2000 BeOpen.com. Copyright © 1995-2001 CNRI. Copyright © 1991-1995 SMC.\0"
+            VALUE "OriginalFilename", PYTHON_DLL_NAME "\0"
+            VALUE "ProductName", "Python\0"
+            VALUE "ProductVersion", PYTHON_VERSION
+        END
+    END
+    BLOCK "VarFileInfo"
+    BEGIN
+        VALUE "Translation", 0x0, 1200
+    END
+END