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.h

@@ -0,0 +1,39 @@
+/*
+** JNetLib
+** Copyright (C) 2000-2001 Nullsoft, Inc.
+** Author: Justin Frankel
+** File: util.h - JNL interface for basic network utilities
+** License: see jnetlib.h
+**
+** routines you may be interested in:
+**   JNL::open_socketlib(); 
+**    opens the socket library. Call this once before using any network
+**    code. If you create a new thread, call this again. Only really an
+**    issue for Win32 support, but use it anyway for portability/
+**
+**   JNL::close_Socketlib();
+**    closes the socketlib. Call this when you're done with the network,
+**    after all your JNetLib objects have been destroyed.
+**
+**   unsigned int JNL::ipstr_to_addr(const char *cp); 
+**    gives you the integer representation of a ip address in dotted 
+**    decimal form.
+**
+**  JNL::addr_to_ipstr(unsigned int addr, char *host, int maxhostlen);
+**    gives you the dotted decimal notation of an integer ip address.
+**
+*/
+
+#ifndef _UTIL_H_
+#define _UTIL_H_
+
+class JNL
+{
+  public:
+    static int open_socketlib();
+    static void close_socketlib();
+    static unsigned int ipstr_to_addr(const char *cp);
+    static void addr_to_ipstr(unsigned int addr, char *host, int maxhostlen);
+};
+
+#endif //_UTIL_H_