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_printf.h

@@ -0,0 +1,27 @@
+#ifndef _WDL_WIN32_PRINTF_H_
+#define _WDL_WIN32_PRINTF_H_
+
+#include <windows.h>
+#ifdef printf
+#undef printf
+#endif
+#define printf wdl_printf
+
+// this file is designed to be temporarily included when printf() debugging on win32
+
+
+static void wdl_printf(const char *format, ...)
+{
+  char tmp[3800];
+  int rv;
+  va_list va;
+  va_start(va,format);
+  tmp[0]=0;
+  rv=_vsnprintf(tmp,sizeof(tmp),format,va); // returns -1  if over, and does not null terminate, ugh
+  va_end(va);
+
+  if (rv < 0 || rv>=(int)sizeof(tmp)-1) tmp[sizeof(tmp)-1]=0; 
+  OutputDebugStringA(tmp);
+}
+
+#endif