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/rng.h

@@ -0,0 +1,42 @@
+/*
+    WDL - rng.h
+    Copyright (C) 2005 and later, Cockos Incorporated
+   
+    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.
+    
+*/
+
+/*
+
+  This header provides the interface to a decent random number generator, 
+  that internally uses a 256-bit state, and SHA-1 to iterate. We wouldn't consider
+  this RNG to be cryptographically secure, but it may be decent.
+
+*/
+
+#ifndef _WDL_RNG_H_
+#define _WDL_RNG_H_
+
+
+void WDL_RNG_addentropy(void *buf, int buflen); // add entropy to the RNG
+
+int WDL_RNG_int32(); // get a random integer
+void WDL_RNG_bytes(void *buf, int buflen); // get a random string of bytes
+
+
+#endif
+