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/commands.py

@@ -0,0 +1,90 @@
+"""Execute shell commands via os.popen() and return status, output.
+
+Interface summary:
+
+       import commands
+
+       outtext = commands.getoutput(cmd)
+       (exitstatus, outtext) = commands.getstatusoutput(cmd)
+       outtext = commands.getstatus(file)  # returns output of "ls -ld file"
+
+A trailing newline is removed from the output string.
+
+Encapsulates the basic operation:
+
+      pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')
+      text = pipe.read()
+      sts = pipe.close()
+
+ [Note:  it would be nice to add functions to interpret the exit status.]
+"""
+from warnings import warnpy3k
+warnpy3k("the commands module has been removed in Python 3.0; "
+         "use the subprocess module instead", stacklevel=2)
+del warnpy3k
+
+__all__ = ["getstatusoutput","getoutput","getstatus"]
+
+# Module 'commands'
+#
+# Various tools for executing commands and looking at their output and status.
+#
+# NB This only works (and is only relevant) for UNIX.
+
+
+# Get 'ls -l' status for an object into a string
+#
+def getstatus(file):
+    """Return output of "ls -ld <file>" in a string."""
+    import warnings
+    warnings.warn("commands.getstatus() is deprecated", DeprecationWarning, 2)
+    return getoutput('ls -ld' + mkarg(file))
+
+
+# Get the output from a shell command into a string.
+# The exit status is ignored; a trailing newline is stripped.
+# Assume the command will work with '{ ... ; } 2>&1' around it..
+#
+def getoutput(cmd):
+    """Return output (stdout or stderr) of executing cmd in a shell."""
+    return getstatusoutput(cmd)[1]
+
+
+# Ditto but preserving the exit status.
+# Returns a pair (sts, output)
+#
+def getstatusoutput(cmd):
+    """Return (status, output) of executing cmd in a shell."""
+    import os
+    pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')
+    text = pipe.read()
+    sts = pipe.close()
+    if sts is None: sts = 0
+    if text[-1:] == '\n': text = text[:-1]
+    return sts, text
+
+
+# Make command argument from directory and pathname (prefix space, add quotes).
+#
+def mk2arg(head, x):
+    import os
+    return mkarg(os.path.join(head, x))
+
+
+# Make a shell command argument from a string.
+# Return a string beginning with a space followed by a shell-quoted
+# version of the argument.
+# Two strategies: enclose in single quotes if it contains none;
+# otherwise, enclose in double quotes and prefix quotable characters
+# with backslash.
+#
+def mkarg(x):
+    if '\'' not in x:
+        return ' \'' + x + '\''
+    s = ' "'
+    for c in x:
+        if c in '\\$"`':
+            s = s + '\\'
+        s = s + c
+    s = s + '"'
+    return s