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/tty.py

@@ -0,0 +1,36 @@
+"""Terminal utilities."""
+
+# Author: Steen Lumholt.
+
+from termios import *
+
+__all__ = ["setraw", "setcbreak"]
+
+# Indexes for termios list.
+IFLAG = 0
+OFLAG = 1
+CFLAG = 2
+LFLAG = 3
+ISPEED = 4
+OSPEED = 5
+CC = 6
+
+def setraw(fd, when=TCSAFLUSH):
+    """Put terminal into a raw mode."""
+    mode = tcgetattr(fd)
+    mode[IFLAG] = mode[IFLAG] & ~(BRKINT | ICRNL | INPCK | ISTRIP | IXON)
+    mode[OFLAG] = mode[OFLAG] & ~(OPOST)
+    mode[CFLAG] = mode[CFLAG] & ~(CSIZE | PARENB)
+    mode[CFLAG] = mode[CFLAG] | CS8
+    mode[LFLAG] = mode[LFLAG] & ~(ECHO | ICANON | IEXTEN | ISIG)
+    mode[CC][VMIN] = 1
+    mode[CC][VTIME] = 0
+    tcsetattr(fd, when, mode)
+
+def setcbreak(fd, when=TCSAFLUSH):
+    """Put terminal into a cbreak mode."""
+    mode = tcgetattr(fd)
+    mode[LFLAG] = mode[LFLAG] & ~(ECHO | ICANON)
+    mode[CC][VMIN] = 1
+    mode[CC][VTIME] = 0
+    tcsetattr(fd, when, mode)