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/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