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/Synth/SynthVoice.h

@@ -0,0 +1,99 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+#include "Wavetable.h"
+#include "Oscillator.h"
+#include "SubOscillator.h"
+#include "NoiseOscillator.h"
+#include "FilterBank.h"
+#include "Envelope.h"
+#include "ModulationMatrix.h"
+#include "MacroControls.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Everything a voice needs to render one block. Global (non per-voice) values
+// are provided by the engine; the voice adds its own per-voice modulation.
+// ===========================================================================
+struct RenderContext
+{
+    double sampleRate = 44100.0;
+    const WavetableLibrary* wavetables = nullptr;
+
+    float lfoValues[kNumLfos]      { 0.0f, 0.0f, 0.0f, 0.0f };
+    float macroValues[kNumMacros]  { 0.0f, 0.0f, 0.0f, 0.0f };
+    float modWheel = 0.0f;
+    float pitchBend = 0.0f;
+    float pitchBendRange = 2.0f;
+
+    const ModulationMatrix* matrix = nullptr;
+    const MacroControls*    macros = nullptr;
+
+    OscParams oscA;
+    OscParams oscB;
+
+    bool  subOn = false;
+    int   subShape = 0;
+    int   subOct = -1;
+    float subLevel = 0.0f;
+
+    bool  noiseOn = false;
+    int   noiseType = 0;
+    float noiseLevel = 0.0f;
+
+    FilterBankParams filters;
+
+    float envAttack[4] { 0.01f, 0.01f, 0.01f, 0.01f };
+    float envDecay[4]  { 0.2f, 0.2f, 0.2f, 0.2f };
+    float envSustain[4]{ 0.7f, 0.7f, 0.7f, 0.7f };
+    float envRelease[4]{ 0.3f, 0.3f, 0.3f, 0.3f };
+    float envCurve[4]  { 0.5f, 0.5f, 0.5f, 0.5f };
+};
+
+// ===========================================================================
+// One synthesis voice: two oscillators + sub + noise -> filter bank -> amp
+// envelope. Four envelopes provide per-voice modulation sources.
+// ===========================================================================
+class SynthVoice
+{
+public:
+    void prepare (double sampleRate, int maxBlockSize);
+    void reset();
+
+    void noteOn (int noteNumber, float velocity, double freqHz, juce::uint32 seed);
+    void noteOff();
+
+    void render (float* outL, float* outR, int numSamples, const RenderContext& ctx) noexcept;
+
+    bool isActive() const noexcept     { return active; }
+    bool isReleased() const noexcept   { return released; }
+    int  getNote() const noexcept      { return note; }
+    juce::uint64 getNoteId() const noexcept { return noteId; }
+    float getModulationValue (ModSource source) const noexcept;
+
+private:
+    Oscillator     oscA, oscB;
+    SubOscillator  sub;
+    NoiseOscillator noise;
+    FilterBank     filters;
+    std::array<Envelope, 4> env;
+
+    int    note = -1;
+    float  velocity = 0.0f;
+    double baseFreq = 0.0;
+    bool   active = false;
+    bool   released = false;
+    juce::uint64 noteId = 0;
+
+    bool oscillatorsNeedNoteOn = false;
+
+    juce::uint32 seed = 0;
+    float noteRandom = 0.5f;
+
+    juce::AudioBuffer<float> scratch;   // 2 channels
+};
+
+} // namespace serum