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/FilterBank.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+#include "Filter.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Snapshot of the two-slot filter section.
+// ===========================================================================
+struct FilterBankParams
+{
+    bool  f1On = false, f2On = false;
+    int   f1Type = 0, f2Type = 0;
+    float f1Cutoff = 0.5f, f2Cutoff = 0.5f;
+    float f1Res = 0.0f, f2Res = 0.0f;
+    float f1Drive = 0.0f, f2Drive = 0.0f;
+    float f1Key = 0.0f, f2Key = 0.0f;
+    int   f1Slope = 2, f2Slope = 2;
+    int   route = 0;
+    float mix   = 0.5f;
+    float out   = 1.0f;
+};
+
+// ===========================================================================
+// Two filter slots with serial / parallel / split routing, processed stereo.
+// ===========================================================================
+class FilterBank
+{
+public:
+    void prepare (double sampleRate);
+    void reset();
+
+    void process (float* l, float* r, int numSamples, const FilterBankParams& p, float noteHz) noexcept;
+
+private:
+    Filter f1L, f1R, f2L, f2R;
+
+    static void applySlot (Filter& f, float* buf, int n, bool on, int type, float cutoff,
+                           float res, float drive, float key, int slope, float noteHz) noexcept;
+};
+
+} // namespace serum