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/Engine/Engine.h

@@ -0,0 +1,86 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include <string_view>
+#include <unordered_map>
+#include "Params.h"
+#include "Wavetable.h"
+#include "SynthVoice.h"
+#include "LFO.h"
+#include "FXProcessor.h"
+#include "ModulationMatrix.h"
+#include "MacroControls.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// The synthesis engine: a fixed pool of voices, four global LFOs, a wavetable
+// library and the FX rack. Owns all per-block DSP and MIDI handling.
+// ===========================================================================
+class Engine
+{
+public:
+    Engine();
+    void prepare (double sampleRate, int blockSize, juce::AudioProcessorValueTreeState& apvts);
+    void reset();
+
+    // Ask for the DSP state to be cleared before the next block is rendered.
+    // Safe to call from the message thread while controlLock is held: the audio
+    // thread performs the reset in captureControls, so the DSP objects are only
+    // ever touched by the audio thread.
+    void requestReset() noexcept;
+
+    void processBlock (juce::AudioBuffer<float>& buffer,
+                       juce::MidiBuffer& midi,
+                       juce::AudioProcessorValueTreeState& apvts,
+                       juce::AudioPlayHead* playhead);
+
+    // MIDI
+    void noteOn (int noteNumber, float velocity01);
+    void noteOff (int noteNumber);
+    void allNotesOff();
+
+    // Control-state accessors: hold getControlLock() while reading or editing.
+    const juce::CriticalSection& getControlLock() const { return controlLock; }
+    ModulationMatrix& getMatrix() { return matrix; }
+    MacroControls& getMacros() { return macros; }
+    const std::array<LFO, kNumLfos>& getLfos() const { return controlLfos; }
+    const WavetableLibrary& getWavetables() const { return wavetables; }
+    void setLfoShapeData (int index, const std::vector<float>& data, int steps);
+
+private:
+    std::array<SynthVoice, kNumVoices> voices;
+    std::array<LFO, kNumLfos> lfos, controlLfos;
+    WavetableLibrary wavetables;
+    FXProcessor fx;
+    ModulationMatrix matrix, audioMatrix;
+    MacroControls macros, audioMacros;
+    juce::CriticalSection controlLock;
+    bool resetRequested = false;   // guarded by controlLock
+
+    struct ParameterValue
+    {
+        std::atomic<float>* source;
+        float value;
+    };
+    std::unordered_map<std::string_view, ParameterValue> parameterValues;
+
+    double sr = 44100.0;
+    int    blockSize = 512;
+    double bpm = 120.0;
+    float  pitchBend = 0.0f;
+    float  modWheel  = 0.0f;
+    juce::uint64 noteCounter = 0;
+
+    juce::AudioBuffer<float> mixBuffer;
+
+    SynthVoice* findFreeVoice();
+    SynthVoice* stealVoice();
+    void captureControls();
+    float v (const char* id) const;
+    int vic (const char* id, int maxValue) const;
+    void readOscParams (const paramIds::Oscillator& ids, OscParams& out) const;
+};
+
+} // namespace serum