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/Envelope.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <JuceHeader.h>
+
+namespace serum
+{
+
+// Curve-to-shape mapping shared by the envelope DSP and the GUI preview, so the
+// two cannot drift apart.
+inline float attackShapeFor (float curve) noexcept { return 0.3f + curve * 2.7f; }  // 0.3 .. 3.0
+inline float decayShapeFor  (float curve) noexcept { return 3.0f - curve * 2.7f; }  // 3.0 .. 0.3
+
+// ===========================================================================
+// ADSR envelope with curve tension. Durations are in seconds; the sustain
+// level is 0..1. Retriggering (noteOn while active) starts from the current
+// value for click-free legato behaviour.
+// ===========================================================================
+class Envelope
+{
+public:
+    void prepare (double sampleRate) { sr = sampleRate; reset(); }
+    void reset();
+
+    void noteOn();
+    void noteOff();
+
+    void setParams (float attackSec, float decaySec, float sustain, float releaseSec, float curve);
+
+    float process() noexcept;       // advance one sample and return the value
+    float getValue() const noexcept { return value; }
+    bool  isActive() const noexcept { return stage != Stage::Idle; }
+
+    // Snapshot for GUI rendering.
+    struct Shape { float attack = 0, decay = 0, sustain = 0, release = 0, curve = 0.5f; };
+    Shape getShape() const noexcept { return shape; }
+
+private:
+    enum class Stage { Idle, Attack, Decay, Sustain, Release };
+
+    Stage  stage = Stage::Idle;
+    double sr = 44100.0;
+
+    float  value = 0.0f;
+    float  startValue = 0.0f, endValue = 0.0f;
+    double elapsed = 0.0, duration = 0.0;
+    double attackShape = 1.0, decayShape = 1.0;
+
+    float  sustain = 0.7f;
+    double attackSamples = 0.0, decaySamples = 0.0, releaseSamples = 0.0;
+    Shape  shape;
+};
+
+} // namespace serum