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/Include/patchlevel.h

@@ -0,0 +1,43 @@
+
+/* Newfangled version identification scheme.
+
+   This scheme was added in Python 1.5.2b2; before that time, only PATCHLEVEL
+   was available.  To test for presence of the scheme, test for
+   defined(PY_MAJOR_VERSION).
+
+   When the major or minor version changes, the VERSION variable in
+   configure.ac must also be changed.
+
+   There is also (independent) API version information in modsupport.h.
+*/
+
+/* Values for PY_RELEASE_LEVEL */
+#define PY_RELEASE_LEVEL_ALPHA	0xA
+#define PY_RELEASE_LEVEL_BETA	0xB
+#define PY_RELEASE_LEVEL_GAMMA	0xC     /* For release candidates */
+#define PY_RELEASE_LEVEL_FINAL	0xF	/* Serial should be 0 here */
+					/* Higher for patch releases */
+
+/* Version parsed out into numeric values */
+/*--start constants--*/
+#define PY_MAJOR_VERSION	2
+#define PY_MINOR_VERSION	7
+#define PY_MICRO_VERSION	18
+#define PY_RELEASE_LEVEL	PY_RELEASE_LEVEL_FINAL
+#define PY_RELEASE_SERIAL	0
+
+/* Version as a string */
+#define PY_VERSION      	"2.7.18"
+/*--end constants--*/
+
+/* Subversion Revision number of this file (not of the repository). Empty
+   since Mercurial migration. */
+#define PY_PATCHLEVEL_REVISION  ""
+
+/* Version as a single 4-byte hex number, e.g. 0x010502B2 == 1.5.2b2.
+   Use this for numeric comparisons, e.g. #if PY_VERSION_HEX >= ... */
+#define PY_VERSION_HEX ((PY_MAJOR_VERSION << 24) | \
+			(PY_MINOR_VERSION << 16) | \
+			(PY_MICRO_VERSION <<  8) | \
+			(PY_RELEASE_LEVEL <<  4) | \
+			(PY_RELEASE_SERIAL << 0))