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/RISCOS/sleep.c

@@ -0,0 +1,41 @@
+#include "oslib/osmodule.h"
+#include <stdio.h>
+#include "kernel.h"
+#include <limits.h>
+#include <errno.h>
+#include "oslib/taskwindow.h"
+#include "Python.h"
+
+
+int riscos_sleep(double delay)
+{
+    os_t starttime, endtime, time; /* monotonic times (centiseconds) */
+    int *pollword, ret;
+    osbool claimed;
+
+    /* calculate end time */
+    starttime = os_read_monotonic_time();
+    if (starttime + 100.0*delay >INT_MAX)
+        endtime = INT_MAX;
+    else
+        endtime = (os_t)(starttime + 100.0*delay);
+
+    /* allocate (in RMA) and set pollword for xupcall_sleep */
+    pollword = osmodule_alloc(4);
+    *pollword = 1;
+
+    time = starttime;
+    ret = 0;
+    while ( time<endtime && time>=starttime ) {
+        xupcall_sleep (pollword, &claimed);
+        if (PyErr_CheckSignals()) {
+            ret = 1;
+            break;
+        }
+        time = os_read_monotonic_time();
+    }
+
+    /* deallocate pollword */
+    osmodule_free(pollword);
+    return ret;
+}