Add delay and reverb FX section with full DSP

Rename the DIST section to FX and expand it with tempo-synced delay
(Time/Strength/Sync) and Schroeder reverb (Roomsize/Strength/Diffusion).
New FxProcessor runs post-voice on the stereo bus; widen the editor to
1280px to fit the new section. Add a TestHost check that the FX tail
rings out after note release.

Source/dsp/FxProcessor.h

@@ -0,0 +1,71 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include <vector>
+
+namespace monostep
+{
+
+// Tempo-synced delay + Schroeder reverb, applied after the voice.
+class FxProcessor
+{
+public:
+    FxProcessor() = default;
+
+    void prepare (double sampleRate, int numChannels);
+    void reset();
+
+    struct Params
+    {
+        float  delayTime  = 0.35f;   // normalized knob 0..1
+        float  delayMix   = 0.0f;    // 0..1 dry/wet
+        bool   delaySync  = true;    // tempo-synced vs. free milliseconds
+        float  reverbRoom = 0.5f;    // 0..1
+        float  reverbMix  = 0.0f;    // 0..1 dry/wet
+        float  reverbDiff = 0.6f;    // 0..1 allpass diffusion
+        double bpm        = 120.0;
+    };
+
+    void setParams (const Params& p);
+
+    void process (juce::AudioBuffer<float>& buffer, int numSamples);
+
+    // Shared knob-to-time mapping so the editor displays the same values the DSP uses.
+    static float delayTimeSeconds (float knobNormalized, bool sync, double bpm);
+    static juce::String delayTimeLabel (float knobNormalized, bool sync, double bpm);
+
+private:
+    struct DelayChannel
+    {
+        std::vector<float> memory;
+        int writeIndex = 0;
+        float smoothedDelay = 0.0f;
+        float feedbackLp = 0.0f;
+    };
+
+    struct ReverbChannel
+    {
+        std::vector<float> combs[4];
+        std::vector<float> allpasses[2];
+        int combIndex[4] = {};
+        int allpassIndex[2] = {};
+    };
+
+    void processDelay (juce::AudioBuffer<float>& buffer, int numSamples);
+    void processReverb (juce::AudioBuffer<float>& buffer, int numSamples);
+
+    double sampleRate = 44100.0;
+    int numChannels = 2;
+    int maxDelaySamples = 0;
+
+    int combDelaySamples[2][4] = {};
+    int allpassDelaySamples[2][2] = {};
+
+    Params params;
+    float currentDelaySeconds = 0.0f;
+
+    std::vector<DelayChannel> delays;
+    std::vector<ReverbChannel> reverbs;
+};
+
+} // namespace monostep