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/this.py

@@ -0,0 +1,28 @@
+s = """Gur Mra bs Clguba, ol Gvz Crgref
+
+Ornhgvshy vf orggre guna htyl.
+Rkcyvpvg vf orggre guna vzcyvpvg.
+Fvzcyr vf orggre guna pbzcyrk.
+Pbzcyrk vf orggre guna pbzcyvpngrq.
+Syng vf orggre guna arfgrq.
+Fcnefr vf orggre guna qrafr.
+Ernqnovyvgl pbhagf.
+Fcrpvny pnfrf nera'g fcrpvny rabhtu gb oernx gur ehyrf.
+Nygubhtu cenpgvpnyvgl orngf chevgl.
+Reebef fubhyq arire cnff fvyragyl.
+Hayrff rkcyvpvgyl fvyraprq.
+Va gur snpr bs nzovthvgl, ershfr gur grzcgngvba gb thrff.
+Gurer fubhyq or bar-- naq cersrenoyl bayl bar --boivbhf jnl gb qb vg.
+Nygubhtu gung jnl znl abg or boivbhf ng svefg hayrff lbh'er Qhgpu.
+Abj vf orggre guna arire.
+Nygubhtu arire vf bsgra orggre guna *evtug* abj.
+Vs gur vzcyrzragngvba vf uneq gb rkcynva, vg'f n onq vqrn.
+Vs gur vzcyrzragngvba vf rnfl gb rkcynva, vg znl or n tbbq vqrn.
+Anzrfcnprf ner bar ubaxvat terng vqrn -- yrg'f qb zber bs gubfr!"""
+
+d = {}
+for c in (65, 97):
+    for i in range(26):
+        d[chr(i+c)] = chr((i+13) % 26 + c)
+
+print "".join([d.get(c, c) for c in s])