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/jpeglib/jconfig.h

@@ -0,0 +1,15 @@
+#define HAVE_PROTOTYPES
+#define HAVE_STDLIB_H
+#define HAVE_UNSIGNED_SHORT
+#define HAVE_UNSIGNED_CHAR
+#ifndef _WIN32
+#define NEED_SYS_TYPES_H
+#define HAVE_STDDEF_H
+#else
+#include <windows.h>
+#define XMD_H
+#undef FAR
+#define HAVE_BOOLEAN
+typedef short INT16;
+#endif
+