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/win32_helpers.h

@@ -0,0 +1,26 @@
+#ifndef _WDL_WIN32_HELPERS_H_
+#define _WDL_WIN32_HELPERS_H_
+
+
+static WDL_STATICFUNC_UNUSED HMENU InsertSubMenu(HMENU hMenu, int pos, const char *name, int flags=0)
+{
+  HMENU sub = NULL;
+  if (hMenu) 
+  {
+    sub = CreatePopupMenu();
+    InsertMenu(hMenu,pos,flags|MF_BYPOSITION|(sub?MF_POPUP:0)|MF_STRING,(UINT_PTR)sub, name);
+  }
+  return sub;
+}
+
+static WDL_STATICFUNC_UNUSED void InsertMenuString(HMENU hMenu, int pos, const char *name, int idx, int flags=0)
+{
+  if (hMenu) InsertMenu(hMenu,pos,flags|MF_BYPOSITION|MF_STRING, idx, name);
+}
+
+static WDL_STATICFUNC_UNUSED void InsertMenuSeparator(HMENU hMenu, int pos)
+{
+  if (hMenu) InsertMenu(hMenu,pos,MF_BYPOSITION|MF_SEPARATOR, 0, NULL);
+}
+
+#endif