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/Modules/rotatingtree.h

@@ -0,0 +1,27 @@
+/* "Rotating trees" (Armin Rigo)
+ *
+ * Google "splay trees" for the general idea.
+ *
+ * It's a dict-like data structure that works best when accesses are not
+ * random, but follow a strong pattern.  The one implemented here is for
+ * access patterns where the same small set of keys is looked up over
+ * and over again, and this set of keys evolves slowly over time.
+ */
+
+#include <stdlib.h>
+
+#define EMPTY_ROTATING_TREE       ((rotating_node_t *)NULL)
+
+typedef struct rotating_node_s rotating_node_t;
+typedef int (*rotating_tree_enum_fn) (rotating_node_t *node, void *arg);
+
+struct rotating_node_s {
+	void *key;
+	rotating_node_t *left;
+	rotating_node_t *right;
+};
+
+void RotatingTree_Add(rotating_node_t **root, rotating_node_t *node);
+rotating_node_t* RotatingTree_Get(rotating_node_t **root, void *key);
+int RotatingTree_Enum(rotating_node_t *root, rotating_tree_enum_fn enumfn,
+		      void *arg);