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/Modulation/MacroControls.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// One assignable destination for a macro knob.
+// ===========================================================================
+struct MacroAssignment
+{
+    ModTarget target = ModTarget::None;
+    float     depth  = 0.0f;
+};
+
+// ===========================================================================
+// Four macros (Energy, Width, Drive, Atmosphere). Macro *values* are APVTS
+// parameters; this class stores the assignable destinations and serialises
+// them with the preset.
+// ===========================================================================
+class MacroControls
+{
+public:
+    static constexpr int kMaxAssignments = 8;
+
+    std::array<std::vector<MacroAssignment>, kNumMacros> assignments;
+
+    bool addAssignment (int macro, ModTarget target, float depth);
+    void removeAssignment (int macro, int index);
+    void clear();
+
+    juce::ValueTree toValueTree() const;
+    void fromValueTree (const juce::ValueTree& tree);
+
+    static juce::String macroName (int index);
+};
+
+} // namespace serum