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/fnv64.h

@@ -0,0 +1,17 @@
+#ifndef _WDL_FNV64_H_
+#define _WDL_FNV64_H_
+
+#include "wdltypes.h"
+
+#define WDL_FNV64_IV WDL_UINT64_CONST(0xCBF29CE484222325)
+
+static WDL_STATICFUNC_UNUSED WDL_UINT64 WDL_FNV64(WDL_UINT64 h, const unsigned char* data, int sz)
+{
+  for (int i=0; i < sz; ++i)
+  {
+    h *= WDL_UINT64_CONST(0x00000100000001B3);
+    h ^= data[i];
+  }
+  return h;
+}
+#endif