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/LFO.h

@@ -0,0 +1,74 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Free-running LFO with tempo sync, seven shapes (sine/tri/saw/square/S&H/
+// step-sequencer/freehand), adjustable phase, fade-in and start delay.
+// ===========================================================================
+class LFO
+{
+public:
+    static constexpr int kShapePoints = 64;
+
+    LFO() { reset(); }
+    void prepare (double sampleRate) { sr = sampleRate; reset(); }
+    void reset();
+
+    void setParams (float rateNorm, bool sync, float beat, int shape, float phase,
+                    float fade, float delay);
+    void setTempo (double bpm) noexcept { tempo = bpm; }
+    void setShapeData (const std::vector<float>& data, int steps);
+
+    float process() noexcept;                 // advance and return current value
+    float advance (int numSamples) noexcept;
+    float getPhase() const noexcept { return (float) (phase + phaseOffset - std::floor (phase + phaseOffset)); }
+    float getValue() const noexcept { return value; }
+
+    const std::vector<float>& getShapeData() const noexcept { return shapeBuffer; }
+    int   getShapeSteps() const noexcept { return shapeSteps; }
+
+private:
+    double sr = 44100.0;
+    double tempo = 120.0;
+
+    double phase = 0.0;
+    double phaseOffset = 0.0;
+    double rateHz = 1.0;
+    float  value = 0.0f;
+
+    int    shape = 0;
+    bool   sync = false;
+    float  beat = 0.25f;
+
+    double delaySeconds = 0.0;
+    double fadeSeconds  = 0.0;
+    double delayCounter = 0.0;   // samples remaining before start
+    double fadeCounter  = 0.0;   // samples elapsed in fade
+    float  fadeVal      = 1.0f;
+    float  prevDelayParam = -1.0f;
+    float  prevFadeParam  = -1.0f;
+
+    std::vector<float> shapeBuffer;
+    int   shapeSteps = 16;
+
+    juce::Random rng;
+    float  holdValue = 0.0f;
+
+    float shapeValue() noexcept;
+};
+
+// ===========================================================================
+// Pure shape evaluation shared by the audio render (LFO::shapeValue) and the
+// GUI preview (LFODisplay::valueAt), so the drawn shape cannot drift from the
+// played one. `sampleHoldValue` supplies the current held level for
+// LfoShape::SampleHold, which is stateful in the audio path.
+// ===========================================================================
+float lfoShapeValue (float phase, LfoShape shape, int steps,
+                     const float* data, int dataSize, float sampleHoldValue) noexcept;
+
+} // namespace serum