refactor(source): move every implementation into its module directory

- 36 files move into the module whose AGENTS.md already claimed them:
  Engine/, Synth/, Modulation/, Params/, Plugin/, GUI/, EffectUnits/, Resources/
- Source/engine, modulation, params, plugin and synth become Engine,
  Modulation, Params, Plugin and Synth, matching the directories that already
  hold code
- ExactCompare.h stays at the Source/ root as the one shared header
- SERUMALT_INCLUDE_DIRS adds one include directory per module, so the 31
  parent-relative includes become plain names with no path prefix
- the module AGENTS.md files list owned files instead of describing a move
  that had not happened, and README.md and AGENTS.md describe the real tree

Verified: the harness passes CTest, the Linux VST3 and Standalone link, the
Windows cross-build passes its module checks, each configure exposes only its
own targets, no first-party warning line appears, and the generated Windows
manifest is unchanged.

Source/EffectUnits/AGENTS.md

@@ -2,7 +2,7 @@
 
 ## Owned files
 
-- Related rack implementation: `../FXProcessor.h`, `../FXProcessor.cpp`
+- FXProcessor.h, FXProcessor.cpp
 - Biquad.h
 - Hyper.h, Hyper.cpp
 - Chorus.h, Chorus.cpp
@@ -27,7 +27,7 @@
 ## IF-THEN
 
 - IF a unit needs a delay line THEN allocate it in `prepare()` at the full required length and only read and write it in `process`.
-- Include the existing rack interface via `../FXProcessor.h`; no source relocation is implied by this guidance.
+- Include the rack interface via `FXProcessor.h`.
 
 ## Examples
 

Source/EffectUnits/Chorus.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {

Source/EffectUnits/Compressor.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {

Source/EffectUnits/Delay.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {

Source/EffectUnits/Distortion.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 #include "Biquad.h"
 
 namespace serum

Source/EffectUnits/EQ.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 #include "Biquad.h"
 
 namespace serum

Source/EffectUnits/FXProcessor.cpp

@@ -0,0 +1,113 @@
+#include "FXProcessor.h"
+#include "EffectUnits/Hyper.h"
+#include "EffectUnits/Chorus.h"
+#include "EffectUnits/Flanger.h"
+#include "EffectUnits/Phaser.h"
+#include "EffectUnits/Distortion.h"
+#include "EffectUnits/EQ.h"
+#include "EffectUnits/Compressor.h"
+#include "EffectUnits/Delay.h"
+#include "EffectUnits/Reverb.h"
+
+namespace serum
+{
+
+FXProcessor::FXProcessor()
+{
+    for (auto& slotUnits : units)
+    {
+        slotUnits[0] = std::make_unique<HyperUnit>();
+        slotUnits[1] = std::make_unique<ChorusUnit>();
+        slotUnits[2] = std::make_unique<FlangerUnit>();
+        slotUnits[3] = std::make_unique<PhaserUnit>();
+        slotUnits[4] = std::make_unique<DistortionUnit>();
+        slotUnits[5] = std::make_unique<EQUnit>();
+        slotUnits[6] = std::make_unique<CompressorUnit>();
+        slotUnits[7] = std::make_unique<DelayUnit>();
+        slotUnits[8] = std::make_unique<ReverbUnit>();
+    }
+}
+
+FXProcessor::~FXProcessor() = default;
+
+void FXProcessor::prepare (double sampleRate, int maxBlockSize)
+{
+    for (auto& slotUnits : units)
+        for (auto& unit : slotUnits)
+            unit->prepare (sampleRate, maxBlockSize);
+    wet.setSize (2, maxBlockSize, false, false, true);
+    reset();
+}
+
+void FXProcessor::reset()
+{
+    for (auto& slotUnits : units)
+        for (auto& unit : slotUnits)
+            unit->reset();
+    activeTypes.fill ((int) FxType::Off);
+    wet.clear();
+}
+
+juce::String FXProcessor::fxTypeName (int type)
+{
+    switch ((FxType) type)
+    {
+        case FxType::Off:        return "Off";
+        case FxType::Hyper:      return "Hyper";
+        case FxType::Chorus:     return "Chorus";
+        case FxType::Flanger:    return "Flanger";
+        case FxType::Phaser:     return "Phaser";
+        case FxType::Distortion: return "Distortion";
+        case FxType::EQ:         return "EQ";
+        case FxType::Compressor: return "Compressor";
+        case FxType::Delay:      return "Delay";
+        case FxType::Reverb:     return "Reverb";
+        case FxType::Count:      return {};
+        default:                 return {};
+    }
+}
+
+void FXProcessor::process (juce::AudioBuffer<float>& buffer, const FxSlotParams* slots, int numSlots)
+{
+    // The rack is stereo end to end; a mono buffer would read a channel that is not there.
+    jassert (buffer.getNumChannels() >= 2);
+    if (buffer.getNumChannels() < 2)
+        return;
+
+    const int n = buffer.getNumSamples();
+    jassert (n <= wet.getNumSamples());
+    if (n <= 0)
+        return;
+
+    for (int s = 0; s < juce::jmin (numSlots, kNumFxSlots); ++s)
+    {
+        const FxSlotParams& slot = slots[s];
+        const int unitIdx = slot.type - 1;
+        if (unitIdx < 0 || unitIdx >= kNumEffectTypes)
+        {
+            activeTypes[(size_t) s] = (int) FxType::Off;
+            continue;
+        }
+
+        auto& unit = *units[(size_t) s][(size_t) unitIdx];
+        if (activeTypes[(size_t) s] != slot.type)
+        {
+            unit.reset();
+            activeTypes[(size_t) s] = slot.type;
+        }
+        wet.copyFrom (0, 0, buffer, 0, 0, n);
+        wet.copyFrom (1, 0, buffer, 1, 0, n);
+        unit.process (wet.getWritePointer (0), wet.getWritePointer (1), n, slot.p);
+
+        const float mix = juce::jlimit (0.0f, 1.0f, slot.mix);
+        for (int ch = 0; ch < 2; ++ch)
+        {
+            float* out = buffer.getWritePointer (ch);
+            const float* effect = wet.getReadPointer (ch);
+            for (int i = 0; i < n; ++i)
+                out[i] = out[i] * (1.0f - mix) + effect[i] * mix;
+        }
+    }
+}
+
+} // namespace serum

Source/EffectUnits/FXProcessor.h

@@ -0,0 +1,56 @@
+#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
+// preconstructed per slot with independent DSP history.
+// ===========================================================================
+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:
+    static constexpr int kNumEffectTypes = (int) FxType::Count - 1;
+    std::array<std::array<std::unique_ptr<FXUnit>, kNumEffectTypes>, kNumFxSlots> units;
+    std::array<int, kNumFxSlots> activeTypes {};
+    juce::AudioBuffer<float> wet;
+};
+
+} // namespace serum

Source/EffectUnits/Flanger.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {

Source/EffectUnits/Hyper.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 #include "Biquad.h"
 
 namespace serum

Source/EffectUnits/Phaser.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {

Source/EffectUnits/Reverb.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "../FXProcessor.h"
+#include "FXProcessor.h"
 
 namespace serum
 {