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/compiler/future.py

@@ -0,0 +1,74 @@
+"""Parser for future statements
+
+"""
+
+from compiler import ast, walk
+
+def is_future(stmt):
+    """Return true if statement is a well-formed future statement"""
+    if not isinstance(stmt, ast.From):
+        return 0
+    if stmt.modname == "__future__":
+        return 1
+    else:
+        return 0
+
+class FutureParser:
+
+    features = ("nested_scopes", "generators", "division",
+                "absolute_import", "with_statement", "print_function",
+                "unicode_literals")
+
+    def __init__(self):
+        self.found = {} # set
+
+    def visitModule(self, node):
+        stmt = node.node
+        for s in stmt.nodes:
+            if not self.check_stmt(s):
+                break
+
+    def check_stmt(self, stmt):
+        if is_future(stmt):
+            for name, asname in stmt.names:
+                if name in self.features:
+                    self.found[name] = 1
+                else:
+                    raise SyntaxError, \
+                          "future feature %s is not defined" % name
+            stmt.valid_future = 1
+            return 1
+        return 0
+
+    def get_features(self):
+        """Return list of features enabled by future statements"""
+        return self.found.keys()
+
+class BadFutureParser:
+    """Check for invalid future statements"""
+
+    def visitFrom(self, node):
+        if hasattr(node, 'valid_future'):
+            return
+        if node.modname != "__future__":
+            return
+        raise SyntaxError, "invalid future statement " + repr(node)
+
+def find_futures(node):
+    p1 = FutureParser()
+    p2 = BadFutureParser()
+    walk(node, p1)
+    walk(node, p2)
+    return p1.get_features()
+
+if __name__ == "__main__":
+    import sys
+    from compiler import parseFile, walk
+
+    for file in sys.argv[1:]:
+        print file
+        tree = parseFile(file)
+        v = FutureParser()
+        walk(tree, v)
+        print v.found
+        print