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/misc.py

@@ -0,0 +1,73 @@
+
+def flatten(tup):
+    elts = []
+    for elt in tup:
+        if isinstance(elt, tuple):
+            elts = elts + flatten(elt)
+        else:
+            elts.append(elt)
+    return elts
+
+class Set:
+    def __init__(self):
+        self.elts = {}
+    def __len__(self):
+        return len(self.elts)
+    def __contains__(self, elt):
+        return elt in self.elts
+    def add(self, elt):
+        self.elts[elt] = elt
+    def elements(self):
+        return self.elts.keys()
+    def has_elt(self, elt):
+        return elt in self.elts
+    def remove(self, elt):
+        del self.elts[elt]
+    def copy(self):
+        c = Set()
+        c.elts.update(self.elts)
+        return c
+
+class Stack:
+    def __init__(self):
+        self.stack = []
+        self.pop = self.stack.pop
+    def __len__(self):
+        return len(self.stack)
+    def push(self, elt):
+        self.stack.append(elt)
+    def top(self):
+        return self.stack[-1]
+    def __getitem__(self, index): # needed by visitContinue()
+        return self.stack[index]
+
+MANGLE_LEN = 256 # magic constant from compile.c
+
+def mangle(name, klass):
+    if not name.startswith('__'):
+        return name
+    if len(name) + 2 >= MANGLE_LEN:
+        return name
+    if name.endswith('__'):
+        return name
+    try:
+        i = 0
+        while klass[i] == '_':
+            i = i + 1
+    except IndexError:
+        return name
+    klass = klass[i:]
+
+    tlen = len(klass) + len(name)
+    if tlen > MANGLE_LEN:
+        klass = klass[:MANGLE_LEN-tlen]
+
+    return "_%s%s" % (klass, name)
+
+def set_filename(filename, tree):
+    """Set the filename attribute to filename on every node in tree"""
+    worklist = [tree]
+    while worklist:
+        node = worklist.pop(0)
+        node.filename = filename
+        worklist.extend(node.getChildNodes())