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/db2val.h

@@ -0,0 +1,22 @@
+#ifndef _WDL_DB2VAL_H_
+#define _WDL_DB2VAL_H_
+
+#include <math.h>
+
+#define TWENTY_OVER_LN10 8.6858896380650365530225783783321
+#define LN10_OVER_TWENTY 0.11512925464970228420089957273422
+#define DB2VAL(x) exp((x)*LN10_OVER_TWENTY)
+
+static inline double VAL2DB(double x)
+{
+  if (x < 0.0000000298023223876953125) return -150.0;
+  double v=log(x)*TWENTY_OVER_LN10;
+  return v<-150.0?-150.0:v;
+}
+
+static inline double VAL2DB_EX(double x, double mindb)
+{
+  return x <= DB2VAL(mindb) ? mindb : (log(x)*TWENTY_OVER_LN10);
+}
+
+#endif