Initial release — InstaGrain granular synthesizer v1.0

8-voice polyphonic granular synth (VST3/AU/LV2) with:
- 128 grain pool per voice, Hann windowing, linear interpolation
- Root note selector, sample rate correction, sustain pedal (CC64)
- Scatter controls, direction modes (Fwd/Rev/PingPong), freeze
- ADSR envelope, global filter (LP/HP/BP), reverb
- Waveform display with grain visualization
- Drag & drop sample loading, full state save/restore
- CI/CD for Windows/macOS/Linux

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Source/GrainEngine.h

@@ -0,0 +1,86 @@
+#pragma once
+#include <JuceHeader.h>
+#include "GrainVoice.h"
+
+class GrainEngine
+{
+public:
+    static constexpr int maxVoices = 8;
+
+    GrainEngine();
+
+    void prepare (double sampleRate, int samplesPerBlock);
+    void processBlock (juce::AudioBuffer<float>& buffer, juce::MidiBuffer& midiMessages);
+    void loadSample (const juce::File& file);
+    void loadSample (const void* data, size_t dataSize, const juce::String& formatName);
+
+    const juce::AudioBuffer<float>& getSampleBuffer() const { return sampleBuffer; }
+    double getSampleRate() const { return currentSampleRate; }
+    bool hasSample() const { return sampleBuffer.getNumSamples() > 0; }
+    juce::String getSamplePath() const { return loadedSamplePath; }
+
+    // Root note — which MIDI note the sample represents (default C4 = 60)
+    std::atomic<int>   rootNote     { 60 };
+
+    // Grain parameters (GUI → audio)
+    std::atomic<float> position     { 0.5f };
+    std::atomic<float> grainSizeMs  { 100.0f };
+    std::atomic<float> density      { 10.0f };
+    std::atomic<float> pitchSemitones { 0.0f };
+    std::atomic<float> pan          { 0.0f };
+    std::atomic<float> posScatter   { 0.0f };
+    std::atomic<float> sizeScatter  { 0.0f };
+    std::atomic<float> pitchScatter { 0.0f };
+    std::atomic<float> panScatter   { 0.0f };
+    std::atomic<int>   direction    { 0 };
+    std::atomic<bool>  freeze       { false };
+
+    // ADSR
+    std::atomic<float> attackTime   { 0.01f };
+    std::atomic<float> decayTime    { 0.1f };
+    std::atomic<float> sustainLevel { 1.0f };
+    std::atomic<float> releaseTime  { 0.3f };
+
+    // Filter
+    std::atomic<int>   filterType   { 0 };    // 0=LP, 1=HP, 2=BP
+    std::atomic<float> filterCutoff { 20000.0f };
+    std::atomic<float> filterReso   { 0.707f };
+
+    // Reverb
+    std::atomic<float> reverbSize   { 0.0f };
+    std::atomic<float> reverbDecay  { 0.0f };
+
+    // Master
+    std::atomic<float> masterVolume { 1.0f };
+
+    // VU meter levels (audio → GUI)
+    std::atomic<float> vuLevelL { 0.0f };
+    std::atomic<float> vuLevelR { 0.0f };
+
+    // Grain visualization
+    struct ActiveGrainInfo { int startSample; int lengthSamples; float progress; };
+    std::vector<ActiveGrainInfo> getActiveGrainInfo() const;
+
+private:
+    std::array<GrainVoice, maxVoices> voices;
+    juce::AudioBuffer<float> sampleBuffer;
+    juce::String loadedSamplePath;
+    double currentSampleRate = 44100.0;
+    double sourceSampleRate = 44100.0;
+    int currentBlockSize = 512;
+
+    juce::AudioFormatManager formatManager;
+
+    // Global effects
+    juce::dsp::StateVariableTPTFilter<float> filter;
+    juce::Reverb reverb;
+    juce::Reverb::Parameters reverbParams;
+
+    void handleMidiEvent (const juce::MidiMessage& msg);
+    void handleNoteOff (int note);
+    void syncVoiceParameters();
+
+    // Sustain pedal
+    bool sustainPedalDown = false;
+    std::array<bool, maxVoices> sustainedVoices {};  // voices held by pedal
+};