Add bare-minimum extension-reaper-macos REAPER extension skeleton

C/C++ extension built directly against the vendored Cockos reaper-sdk
headers (no Rust/reaper-rs wrapper layer). Loads, registers a test
action, and confirms via REAPER's console — confirmed working in
REAPER 2026-07-15.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

extension-reaper-macos/vendor/reaper-sdk/WDL/jnetlib/util.cpp

@@ -0,0 +1,38 @@
+/*
+** JNetLib
+** Copyright (C) 2000-2001 Nullsoft, Inc.
+** Author: Justin Frankel
+** File: util.cpp - JNL implementation of basic network utilities
+** License: see jnetlib.h
+*/
+
+#include "netinc.h"
+
+#include "util.h"
+#include "../wdlcstring.h"
+
+int JNL::open_socketlib()
+{
+#ifdef _WIN32
+  WSADATA wsaData;
+  if (WSAStartup(MAKEWORD(1, 1), &wsaData)) return 1;
+#endif
+  return 0;
+}
+void JNL::close_socketlib()
+{
+#ifdef _WIN32
+  WSACleanup();
+#endif
+}
+unsigned int JNL::ipstr_to_addr(const char *cp) 
+{ 
+  return ::inet_addr(cp); 
+}
+
+void JNL::addr_to_ipstr(unsigned int addr, char *host, int maxhostlen) 
+{ 
+  struct in_addr a; a.s_addr=addr;
+  char *p=::inet_ntoa(a);
+  lstrcpyn_safe(host,p?p:"",maxhostlen);
+}