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/lice/test/Controller.h

@@ -0,0 +1,9 @@
+/* Controller */
+
+#import <Cocoa/Cocoa.h>
+
+@interface Controller : NSObject
+{
+}
+-(void)awakeFromNib;
+@end