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/RISCOS/support/!Boot

@@ -0,0 +1,22 @@
+set Python$Dir <Obey$Dir>
+set PythonApp$Path <Obey$Dir>.
+
+IconSprites <Obey$Dir>.!Sprites
+
+<Obey$Dir>.AddToPath Python$Path PythonApp:Lib
+<Obey$Dir>.AddToPath Python$Path PythonApp:Lib.plat-riscos
+<Obey$Dir>.AddToPath Python$Path PythonApp:Lib.site-packages
+set Alias$@RunType_ae5 TaskWindow |"python %%*0|" -name |"Python|" -quit -wimpslot 1248k
+| -display
+set File$Type_ae5 Python
+
+| load modules for 32 bit compatibility
+RMEnsure UtilityModule 3.10 Error This application requires RISC OS 3.10 or later
+RMEnsure UtilityModule 3.70 RMEnsure CallASWI 0.02 RMLoad System:Modules.CallASWI
+RMEnsure UtilityModule 3.70 RMEnsure CallASWI 0.02 Error This application requires CallASWI 0.02 or later
+RMEnsure FPEmulator 4.03 RMLoad System:Modules.FPEmulator
+RMEnsure FPEmulator 4.03 Error This application requires FPEmulator 4.03 or later
+RMEnsure SharedCLibrary 5.17 RMLoad System:Modules.CLib
+RMEnsure SharedCLibrary 5.34 Error This application requires SharedCLibrary 5.34 or later
+
+set Alias$Python Run <Python$Dir>.python23 %*0

extension/third_party/cpython-2.7.18/RISCOS/support/!Run

@@ -0,0 +1,2 @@
+<Obey$Dir>.!Boot
+TaskWindow "python" -name "Python" -quit -display -wimpslot 1248k