Initial commit: InstaLPEQ linear phase EQ plugin

Full-featured linear phase EQ with interactive graphical curve display.
FIR-based processing (8192-tap), 8 parametric bands, multi-platform
CI/CD (Windows/macOS/Linux), InstaDrums visual style.

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

Source/FIREngine.h

@@ -0,0 +1,46 @@
+#pragma once
+#include <JuceHeader.h>
+#include "EQBand.h"
+
+class FIREngine : private juce::Thread
+{
+public:
+    static constexpr int defaultFFTOrder = 13;  // 8192 taps
+    static constexpr int maxBands = 8;
+
+    FIREngine();
+    ~FIREngine() override;
+
+    void start (double sampleRate);
+    void stop();
+
+    // Called from GUI thread
+    void setBands (const std::vector<EQBand>& newBands);
+    void setFFTOrder (int order);
+
+    // Called from audio thread — returns new FIR if available, nullptr otherwise
+    std::unique_ptr<juce::AudioBuffer<float>> getNewFIR();
+
+    // Get magnitude response in dB for display (thread-safe copy)
+    std::vector<float> getMagnitudeResponseDb() const;
+
+    int getFIRLength() const { return 1 << fftOrder.load(); }
+    int getLatencySamples() const { return getFIRLength() / 2; }
+
+private:
+    void run() override;
+    juce::AudioBuffer<float> generateFIR (const std::vector<EQBand>& bands, double sr, int order);
+
+    std::atomic<double> sampleRate { 44100.0 };
+    std::atomic<int> fftOrder { defaultFFTOrder };
+    std::atomic<bool> needsUpdate { true };
+
+    std::vector<EQBand> currentBands;
+    mutable juce::SpinLock bandLock;
+
+    std::unique_ptr<juce::AudioBuffer<float>> pendingFIR;
+    juce::SpinLock firLock;
+
+    std::vector<float> magnitudeDb;
+    mutable juce::SpinLock magLock;
+};