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/main.m

@@ -0,0 +1,15 @@
+//
+//  main.m
+//  swell_myapp
+//
+
+#import <Cocoa/Cocoa.h>
+
+int main(int argc, const char * argv[])
+{
+  extern const char **g_argv;
+  extern int g_argc;
+  g_argc=argc;
+  g_argv=argv;
+  return NSApplicationMain(argc, argv);
+}