feat(fx): add effect rack and effect units

Source/EffectUnits/Delay.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "../FXProcessor.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Delay — stereo delay with feedback, damping and ping-pong. p[0]=time,
+// p[1]=feedback, p[2]=damping, p[3]=ping-pong/width.
+// ===========================================================================
+class DelayUnit : public FXUnit
+{
+public:
+    void prepare (double sampleRate, int maxBlockSize) override;
+    void reset() override;
+    void process (float* l, float* r, int numSamples, const float p[4]) override;
+
+private:
+    double sr = 44100.0;
+    std::vector<float> delayL, delayR;
+    int    writePos = 0;
+    float  dampL = 0.0f, dampR = 0.0f;
+};
+
+} // namespace serum