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/Objects/stringlib/fastsearch.h

@@ -0,0 +1,160 @@
+/* stringlib: fastsearch implementation */
+
+#ifndef STRINGLIB_FASTSEARCH_H
+#define STRINGLIB_FASTSEARCH_H
+
+/* fast search/count implementation, based on a mix between boyer-
+   moore and horspool, with a few more bells and whistles on the top.
+   for some more background, see: http://effbot.org/zone/stringlib.htm */
+
+/* note: fastsearch may access s[n], which isn't a problem when using
+   Python's ordinary string types, but may cause problems if you're
+   using this code in other contexts.  also, the count mode returns -1
+   if there cannot possible be a match in the target string, and 0 if
+   it has actually checked for matches, but didn't find any.  callers
+   beware! */
+
+#define FAST_COUNT 0
+#define FAST_SEARCH 1
+#define FAST_RSEARCH 2
+
+#if LONG_BIT >= 128
+#define STRINGLIB_BLOOM_WIDTH 128
+#elif LONG_BIT >= 64
+#define STRINGLIB_BLOOM_WIDTH 64
+#elif LONG_BIT >= 32
+#define STRINGLIB_BLOOM_WIDTH 32
+#else
+#error "LONG_BIT is smaller than 32"
+#endif
+
+#define STRINGLIB_BLOOM_ADD(mask, ch) \
+    ((mask |= (1UL << ((ch) & (STRINGLIB_BLOOM_WIDTH -1)))))
+#define STRINGLIB_BLOOM(mask, ch)     \
+    ((mask &  (1UL << ((ch) & (STRINGLIB_BLOOM_WIDTH -1)))))
+
+Py_LOCAL_INLINE(Py_ssize_t)
+fastsearch(const STRINGLIB_CHAR* s, Py_ssize_t n,
+           const STRINGLIB_CHAR* p, Py_ssize_t m,
+           Py_ssize_t maxcount, int mode)
+{
+    unsigned long mask;
+    Py_ssize_t skip, count = 0;
+    Py_ssize_t i, j, mlast, w;
+
+    w = n - m;
+
+    if (w < 0 || (mode == FAST_COUNT && maxcount == 0))
+        return -1;
+
+    /* look for special cases */
+    if (m <= 1) {
+        if (m <= 0)
+            return -1;
+        /* use special case for 1-character strings */
+        if (mode == FAST_COUNT) {
+            for (i = 0; i < n; i++)
+                if (s[i] == p[0]) {
+                    count++;
+                    if (count == maxcount)
+                        return maxcount;
+                }
+            return count;
+        } else if (mode == FAST_SEARCH) {
+            for (i = 0; i < n; i++)
+                if (s[i] == p[0])
+                    return i;
+        } else {    /* FAST_RSEARCH */
+            for (i = n - 1; i > -1; i--)
+                if (s[i] == p[0])
+                    return i;
+        }
+        return -1;
+    }
+
+    mlast = m - 1;
+    skip = mlast - 1;
+    mask = 0;
+
+    if (mode != FAST_RSEARCH) {
+
+        /* create compressed boyer-moore delta 1 table */
+
+        /* process pattern[:-1] */
+        for (i = 0; i < mlast; i++) {
+            STRINGLIB_BLOOM_ADD(mask, p[i]);
+            if (p[i] == p[mlast])
+                skip = mlast - i - 1;
+        }
+        /* process pattern[-1] outside the loop */
+        STRINGLIB_BLOOM_ADD(mask, p[mlast]);
+
+        for (i = 0; i <= w; i++) {
+            /* note: using mlast in the skip path slows things down on x86 */
+            if (s[i+m-1] == p[m-1]) {
+                /* candidate match */
+                for (j = 0; j < mlast; j++)
+                    if (s[i+j] != p[j])
+                        break;
+                if (j == mlast) {
+                    /* got a match! */
+                    if (mode != FAST_COUNT)
+                        return i;
+                    count++;
+                    if (count == maxcount)
+                        return maxcount;
+                    i = i + mlast;
+                    continue;
+                }
+                /* miss: check if next character is part of pattern */
+                if (!STRINGLIB_BLOOM(mask, s[i+m]))
+                    i = i + m;
+                else
+                    i = i + skip;
+            } else {
+                /* skip: check if next character is part of pattern */
+                if (!STRINGLIB_BLOOM(mask, s[i+m]))
+                    i = i + m;
+            }
+        }
+    } else {    /* FAST_RSEARCH */
+
+        /* create compressed boyer-moore delta 1 table */
+
+        /* process pattern[0] outside the loop */
+        STRINGLIB_BLOOM_ADD(mask, p[0]);
+        /* process pattern[:0:-1] */
+        for (i = mlast; i > 0; i--) {
+            STRINGLIB_BLOOM_ADD(mask, p[i]);
+            if (p[i] == p[0])
+                skip = i - 1;
+        }
+
+        for (i = w; i >= 0; i--) {
+            if (s[i] == p[0]) {
+                /* candidate match */
+                for (j = mlast; j > 0; j--)
+                    if (s[i+j] != p[j])
+                        break;
+                if (j == 0)
+                    /* got a match! */
+                    return i;
+                /* miss: check if previous character is part of pattern */
+                if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1]))
+                    i = i - m;
+                else
+                    i = i - skip;
+            } else {
+                /* skip: check if previous character is part of pattern */
+                if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1]))
+                    i = i - m;
+            }
+        }
+    }
+
+    if (mode != FAST_COUNT)
+        return -1;
+    return count;
+}
+
+#endif