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/swell/sample_project/MyApp-Prefix.pch

@@ -0,0 +1,9 @@
+//
+//  Prefix header
+//
+//  The contents of this file are implicitly included at the beginning of every source file.
+//
+
+#ifdef __OBJC__
+  #import <Cocoa/Cocoa.h>
+#endif