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/swellappmain.h

@@ -0,0 +1,14 @@
+#import <Cocoa/Cocoa.h>
+
+
+@interface SWELLApplication : NSApplication {
+
+}
+- (void)sendEvent:(NSEvent *)anEvent;
+
+@end
+
+@interface SWELLAppController : NSObject { 
+}
+-(IBAction)onSysMenuCommand:(id)sender;
+@end