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/sinewavegen.h

@@ -0,0 +1,59 @@
+#ifndef _WDL_SINEWAVEGEN_H_
+#define _WDL_SINEWAVEGEN_H_
+
+
+// note: calling new WDL_SineWaveGenerator isnt strictly necessary, you can also do WDL_SineWaveGenerator *gens = (WDL_SineWaveGenerator *)malloc(512*sizeof(WDL_SineWaveGenerator));
+// as long as you call Reset() and SetFreq() it should be fine.
+
+
+// note: won't really work for high frequencies...
+
+class WDL_SineWaveGenerator 
+{
+  double m_lastfreq;
+  double m_mul1,m_mul2;
+  double m_pos,m_vel;
+
+public:
+  WDL_SineWaveGenerator() { Reset(); m_mul1=m_mul2=m_pos=m_vel=0.0; } 
+  ~WDL_SineWaveGenerator() { }
+
+  void Reset() { m_lastfreq=0.0; } // must call this before anything
+
+  void SetFreq(double freq) // be sure to call this before calling Gen(), or on freq change, or after a Reset()
+                            // freq is frequency/(samplerate*0.5) (so 0..1 is valid, though over 0.3 is probably not a good idea)
+  {    
+    freq*=3.1415926535897932384626433832795; // scale to freq*PI
+
+    if (m_lastfreq<=0.0)
+    {
+      m_pos=0.0;
+      m_vel = 1.0/freq;
+    }
+    else
+    {
+      if (freq==m_lastfreq) return;
+      m_vel *= m_lastfreq/freq;
+    }
+    m_lastfreq=freq;
+
+    double tmp2 = 1.0/(1.0+(freq*=freq));
+    m_mul1 = (1.0-freq)*tmp2;
+    m_mul2 = freq*2.0*tmp2;
+  }
+
+  double Gen() // returns sine
+  {
+    double rv=m_pos;
+    m_vel -= rv + (m_pos = rv*m_mul1 + m_vel*m_mul2);
+    return rv;
+  }
+
+  double GetNextCos()  // call BEFORE Gen() if you want the cosine of the next value
+  {
+    return m_vel * m_lastfreq;
+  }
+
+};
+
+#endif