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/atof.c

@@ -0,0 +1,50 @@
+
+/* Just in case you haven't got an atof() around...
+   This one doesn't check for bad syntax or overflow,
+   and is slow and inaccurate.
+   But it's good enough for the occasional string literal... */
+
+#include "pyconfig.h"
+
+#include <ctype.h>
+
+double atof(char *s)
+{
+    double a = 0.0;
+    int e = 0;
+    int c;
+    while ((c = *s++) != '\0' && isdigit(c)) {
+        a = a*10.0 + (c - '0');
+    }
+    if (c == '.') {
+        while ((c = *s++) != '\0' && isdigit(c)) {
+            a = a*10.0 + (c - '0');
+            e = e-1;
+        }
+    }
+    if (c == 'e' || c == 'E') {
+        int sign = 1;
+        int i = 0;
+        c = *s++;
+        if (c == '+')
+            c = *s++;
+        else if (c == '-') {
+            c = *s++;
+            sign = -1;
+        }
+        while (isdigit(c)) {
+            i = i*10 + (c - '0');
+            c = *s++;
+        }
+        e += i*sign;
+    }
+    while (e > 0) {
+        a *= 10.0;
+        e--;
+    }
+    while (e < 0) {
+        a *= 0.1;
+        e++;
+    }
+    return a;
+}