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/syntax.py

@@ -0,0 +1,46 @@
+"""Check for errs in the AST.
+
+The Python parser does not catch all syntax errors.  Others, like
+assignments with invalid targets, are caught in the code generation
+phase.
+
+The compiler package catches some errors in the transformer module.
+But it seems clearer to write checkers that use the AST to detect
+errors.
+"""
+
+from compiler import ast, walk
+
+def check(tree, multi=None):
+    v = SyntaxErrorChecker(multi)
+    walk(tree, v)
+    return v.errors
+
+class SyntaxErrorChecker:
+    """A visitor to find syntax errors in the AST."""
+
+    def __init__(self, multi=None):
+        """Create new visitor object.
+
+        If optional argument multi is not None, then print messages
+        for each error rather than raising a SyntaxError for the
+        first.
+        """
+        self.multi = multi
+        self.errors = 0
+
+    def error(self, node, msg):
+        self.errors = self.errors + 1
+        if self.multi is not None:
+            print "%s:%s: %s" % (node.filename, node.lineno, msg)
+        else:
+            raise SyntaxError, "%s (%s:%s)" % (msg, node.filename, node.lineno)
+
+    def visitAssign(self, node):
+        # the transformer module handles many of these
+        pass
+##        for target in node.nodes:
+##            if isinstance(target, ast.AssList):
+##                if target.lineno is None:
+##                    target.lineno = node.lineno
+##                self.error(target, "can't assign to list comprehension")