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/ModulationMatrix.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// A single modulation connection: source -> target with a static depth and a
+// bipolar flag. One level of modulation only (no modulation of modulation).
+// ===========================================================================
+struct ModConnection
+{
+    ModSource source   = ModSource::Lfo1;
+    ModTarget target   = ModTarget::None;
+    float     depth    = 0.0f;
+    bool      bipolar  = false;
+};
+
+// ===========================================================================
+// Ordered collection of modulation connections. Serialisable to/from a
+// juce::ValueTree so that connections survive preset save/load.
+// ===========================================================================
+class ModulationMatrix
+{
+public:
+    static constexpr int kMaxConnections = 32;
+
+    std::vector<ModConnection> connections;
+
+    bool addConnection (ModSource source, ModTarget target, float depth, bool bipolar);
+    void removeConnection (int index);
+    void removeAllWithTarget (ModTarget target);
+    void clear() { connections.clear(); }
+
+    int  size() const noexcept { return (int) connections.size(); }
+
+    juce::ValueTree toValueTree() const;
+    void fromValueTree (const juce::ValueTree& tree);
+};
+
+} // namespace serum