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/Plugin/PluginProcessor.h

@@ -0,0 +1,72 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include <atomic>
+#include "Params.h"
+#include "Engine.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// SerumAlt — a wavetable synthesiser (VST3 / AU). Hosts the APVTS, the audio
+// engine, preset management and the RAVE toggle.
+// ===========================================================================
+class SerumAltAudioProcessor : public juce::AudioProcessor
+{
+public:
+    SerumAltAudioProcessor();
+    ~SerumAltAudioProcessor() override;
+
+    // --- AudioProcessor ---
+    void prepareToPlay (double sampleRate, int samplesPerBlock) override;
+    void releaseResources() override;
+    void processBlock (juce::AudioBuffer<float>&, juce::MidiBuffer&) override;
+    // Keeps the base class double-precision overload visible, so it is not hidden.
+    using juce::AudioProcessor::processBlock;
+
+    juce::AudioProcessorEditor* createEditor() override;
+    bool hasEditor() const override                 { return true; }
+
+    const juce::String getName() const override     { return "SerumAlt"; }
+    bool acceptsMidi() const override               { return true; }
+    bool producesMidi() const override              { return false; }
+    bool isMidiEffect() const override              { return false; }
+    double getTailLengthSeconds() const override    { return 2.0; }
+
+    int  getNumPrograms() override;
+    int  getCurrentProgram() override;
+    void setCurrentProgram (int index) override;
+    const juce::String getProgramName (int index) override;
+    void changeProgramName (int index, const juce::String& newName) override;
+
+    void getStateInformation (juce::MemoryBlock& destData) override;
+    void setStateInformation (const void* data, int sizeInBytes) override;
+
+    // --- SerumAlt ---
+    void setRaveEnabled (bool enabled);
+    bool isRaveEnabled() const;
+
+    int  getUiScaleIndex() const;
+    void setUiScaleIndex (int index);
+    float getUiScale() const;
+
+    void loadFactoryPreset (int index);
+    int  getNumFactoryPresets() const;
+
+    // Public control state (hold the engine control lock for matrix/macros/LFOs).
+    juce::AudioProcessorValueTreeState parameters;
+    Engine engine;
+
+    static juce::AudioProcessorValueTreeState::ParameterLayout createParameterLayout();
+
+private:
+    std::atomic<int> currentProgram { 0 };
+
+    void restoreLfoShapesFromState (const juce::ValueTree& state);
+    void saveLfoShapesToState (juce::ValueTree& state) const;
+
+    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (SerumAltAudioProcessor)
+};
+
+} // namespace serum