feat(fx): add effect rack and effect units

Source/EffectUnits/EQ.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "../FXProcessor.h"
+#include "Biquad.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// EQ — three-band (low shelf / peak / high shelf). p[0]=low gain, p[1]=mid
+// gain, p[2]=high gain, p[3]=mid frequency.
+// ===========================================================================
+class EQUnit : 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;
+    Biquad lowL, lowR, midL, midR, highL, highR;
+};
+
+} // namespace serum