feat(fx): add effect rack and effect units

Source/FXProcessor.h

@@ -0,0 +1,54 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Base class for a stereo effect unit. Each unit processes a full-wet buffer.
+// ===========================================================================
+class FXUnit
+{
+public:
+    virtual ~FXUnit() = default;
+    virtual void prepare (double sampleRate, int maxBlockSize) = 0;
+    virtual void reset() = 0;
+    virtual void process (float* l, float* r, int numSamples, const float p[4]) = 0;
+};
+
+// ===========================================================================
+// Snapshot of one FX slot.
+// ===========================================================================
+struct FxSlotParams
+{
+    int   type = (int) FxType::Off;
+    float mix  = 0.0f;
+    float p[4] = { 0.0f, 0.0f, 0.0f, 0.0f };
+};
+
+// ===========================================================================
+// Reorderable effects rack. Slots are processed in list order; reordering is
+// simply swapping FxSlotParams entries. Nine effect unit implementations are
+// owned here and shared across slots.
+// ===========================================================================
+class FXProcessor
+{
+public:
+    FXProcessor();
+    ~FXProcessor();
+
+    void prepare (double sampleRate, int maxBlockSize);
+    void reset();
+
+    void process (juce::AudioBuffer<float>& buffer, const FxSlotParams* slots, int numSlots);
+
+    static juce::String fxTypeName (int type);
+
+private:
+    std::array<std::unique_ptr<FXUnit>, 9> units;
+    juce::AudioBuffer<float> dry, wet;
+};
+
+} // namespace serum