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/Python/pystrcmp.c

@@ -0,0 +1,26 @@
+/* Cross platform case insensitive string compare functions
+ */
+
+#include "Python.h"
+
+int
+PyOS_mystrnicmp(const char *s1, const char *s2, Py_ssize_t size)
+{
+    if (size == 0)
+        return 0;
+    while ((--size > 0) &&
+           (tolower((unsigned)*s1) == tolower((unsigned)*s2))) {
+        if (!*s1++ || !*s2++)
+            break;
+    }
+    return tolower((unsigned)*s1) - tolower((unsigned)*s2);
+}
+
+int
+PyOS_mystricmp(const char *s1, const char *s2)
+{
+    while (*s1 && (tolower((unsigned)*s1++) == tolower((unsigned)*s2++))) {
+        ;
+    }
+    return (tolower((unsigned)*s1) - tolower((unsigned)*s2));
+}