feat(modulation): add envelopes, LFOs and modulation matrix

Source/Envelope.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <JuceHeader.h>
+
+namespace serum
+{
+
+// ===========================================================================
+// ADSR envelope with curve tension. Durations are in seconds; the sustain
+// level is 0..1. Retriggering (noteOn while active) starts from the current
+// value for click-free legato behaviour.
+// ===========================================================================
+class Envelope
+{
+public:
+    void prepare (double sampleRate) { sr = sampleRate; reset(); }
+    void reset();
+
+    void noteOn();
+    void noteOff();
+
+    void setParams (float attackSec, float decaySec, float sustain, float releaseSec, float curve);
+
+    float process() noexcept;       // advance one sample and return the value
+    float getValue() const noexcept { return value; }
+    bool  isActive() const noexcept { return stage != Stage::Idle; }
+
+    // Snapshot for GUI rendering.
+    struct Shape { float attack = 0, decay = 0, sustain = 0, release = 0, curve = 0.5f; };
+    Shape getShape() const noexcept { return shape; }
+
+private:
+    enum class Stage { Idle, Attack, Decay, Sustain, Release };
+
+    Stage  stage = Stage::Idle;
+    double sr = 44100.0;
+
+    float  value = 0.0f;
+    float  startValue = 0.0f, endValue = 0.0f;
+    double elapsed = 0.0, duration = 0.0;
+    double attackShape = 1.0, decayShape = 1.0;
+
+    float  sustain = 0.7f;
+    double attackSamples = 0.0, decaySamples = 0.0, releaseSamples = 0.0;
+    Shape  shape;
+};
+
+} // namespace serum