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/giflib/config.h

@@ -0,0 +1,14 @@
+#ifndef _CONFIG_H_
+#define _CONFIG_H_
+#ifdef _WIN32
+#include <windows.h>
+#include <io.h>
+#define _OPEN_BINARY
+#else
+typedef unsigned int UINT32;
+#include <unistd.h>
+#include <string.h>
+#include <stdlib.h>
+#endif
+#include <fcntl.h>
+#endif