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/Lib/dircache.py

@@ -0,0 +1,41 @@
+"""Read and cache directory listings.
+
+The listdir() routine returns a sorted list of the files in a directory,
+using a cache to avoid reading the directory more often than necessary.
+The annotate() routine appends slashes to directories."""
+from warnings import warnpy3k
+warnpy3k("the dircache module has been removed in Python 3.0", stacklevel=2)
+del warnpy3k
+
+import os
+
+__all__ = ["listdir", "opendir", "annotate", "reset"]
+
+cache = {}
+
+def reset():
+    """Reset the cache completely."""
+    global cache
+    cache = {}
+
+def listdir(path):
+    """List directory contents, using cache."""
+    try:
+        cached_mtime, list = cache[path]
+        del cache[path]
+    except KeyError:
+        cached_mtime, list = -1, []
+    mtime = os.stat(path).st_mtime
+    if mtime != cached_mtime:
+        list = os.listdir(path)
+        list.sort()
+    cache[path] = mtime, list
+    return list
+
+opendir = listdir # XXX backward compatibility
+
+def annotate(head, list):
+    """Add '/' suffixes to directories."""
+    for i in range(len(list)):
+        if os.path.isdir(os.path.join(head, list[i])):
+            list[i] = list[i] + '/'