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/dup2.c

@@ -0,0 +1,31 @@
+/*
+ * Public domain dup2() lookalike
+ * by Curtis Jackson @ AT&T Technologies, Burlington, NC
+ * electronic address:  burl!rcj
+ *
+ * dup2 performs the following functions:
+ *
+ * Check to make sure that fd1 is a valid open file descriptor.
+ * Check to see if fd2 is already open; if so, close it.
+ * Duplicate fd1 onto fd2; checking to make sure fd2 is a valid fd.
+ * Return fd2 if all went well; return BADEXIT otherwise.
+ */
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#define BADEXIT -1
+
+int
+dup2(int fd1, int fd2)
+{
+	if (fd1 != fd2) {
+		if (fcntl(fd1, F_GETFL) < 0)
+			return BADEXIT;
+		if (fcntl(fd2, F_GETFL) >= 0)
+			close(fd2);
+		if (fcntl(fd1, F_DUPFD, fd2) < 0)
+			return BADEXIT;
+	}
+	return fd2;
+}