feat(fx): add effect rack and effect units

Source/EffectUnits/Reverb.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "../FXProcessor.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Reverb — Freeverb-style Schroeder reverb (8 combs + 4 allpasses per channel)
+// with predelay. p[0]=size, p[1]=damping, p[2]=width, p[3]=predelay.
+// ===========================================================================
+class ReverbUnit : 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::array<std::vector<float>, 8> combL, combR;
+    std::array<int, 8> combPosL {}, combPosR {};
+    std::array<float, 8> combFiltL {}, combFiltR {};
+
+    std::array<std::vector<float>, 4> apL, apR;
+    std::array<int, 4> apPosL {}, apPosR {};
+
+    std::vector<float> preL, preR;
+    int prePos = 0;
+};
+
+} // namespace serum