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/jnetlib.h

@@ -0,0 +1,47 @@
+/*
+** JNetLib
+** Copyright (C) 2008 Cockos Inc
+** Copyright (C) 2000-2003 Nullsoft, Inc.
+** Author: Justin Frankel
+** File: jnetlib.h - JNL main include file (not really necessary).
+**
+** For documentation, look at the following files:
+**  Generic network initialization: netinc.h
+**  DNS: asyncdns.h
+**  TCP connections: connection.h
+**  HTTP GET connections: httpget.h
+**  TCP listen: listen.h
+**
+**  license: 
+**
+**  This software is provided 'as-is', without any express or implied
+**  warranty.  In no event will the authors be held liable for any damages
+**  arising from the use of this software.
+**
+**  Permission is granted to anyone to use this software for any purpose,
+**  including commercial applications, and to alter it and redistribute it
+**  freely, subject to the following restrictions:
+**
+**  1. The origin of this software must not be misrepresented; you must not
+**     claim that you wrote the original software. If you use this software
+**     in a product, an acknowledgment in the product documentation would be
+**     appreciated but is not required.
+**  2. Altered source versions must be plainly marked as such, and must not be
+**     misrepresented as being the original software.
+**  3. This notice may not be removed or altered from any source distribution.
+**
+*/
+
+#ifndef _JNETLIB_H_
+#define _JNETLIB_H_
+
+#include "netinc.h"
+#include "util.h"
+
+#include "asyncdns.h"
+#include "connection.h"
+#include "httpget.h"
+#include "httpserv.h"
+#include "listen.h"
+
+#endif//_JNETLIB_H_