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/SubOscillator.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Simple sub oscillator: sine or triangle, one/two octaves down or unison.
+// ===========================================================================
+class SubOscillator
+{
+public:
+    void prepare (double sampleRate)  { sr = sampleRate; phase = 0.0; }
+    void reset()                       { phase = 0.0; }
+
+    void noteOn();
+
+    // Accumulate into out (mono; the caller pans/levels it).
+    void processAdd (double freqHz, int shape, float level, float& out) noexcept;
+
+private:
+    double sr    = 44100.0;
+    double phase = 0.0;
+};
+
+} // namespace serum