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/Filter.h

@@ -0,0 +1,61 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Mono filter with 7 models (Ladder LP/HP/BP, Diode, Comb, Formant, Screamer),
+// 6/12/24 dB slopes (ladder family) and per-sample drive + keytrack. All state
+// is clamped to prevent NaN/inf at extreme settings.
+// ===========================================================================
+class Filter
+{
+public:
+    void prepare (double sampleRate);
+    void reset();
+
+    // Process a mono buffer in-place. cutoffNorm is 0..1 (mapped to 20Hz..20kHz).
+    void process (float* samples, int numSamples, float cutoffNorm, float res,
+                  float drive, float keytrack, float noteHz, int type, int slope) noexcept;
+
+    // Single-sample version (used by the comb/naive paths where convenient).
+    float processSample (float in, float cutoffHz, float res, float drive, int type, int slope) noexcept;
+    static float getCutoffHz (float cutoffNorm, float keytrack, float noteHz) noexcept;
+
+private:
+    double sr = 44100.0;
+    float lastCutoffHz = -1.0f, lastRes = -1.0f;
+    int lastType = -1;
+    double ladderG = 0.0;
+
+    // TPT SVF state (also reused by formant/screamer).
+    double ic1eq = 0.0, ic2eq = 0.0;
+    double lastG = 0.0, lastK = 0.0;
+    double a1 = 0.0, a2 = 0.0, a3 = 0.0;
+
+    // Ladder stage state.
+    std::array<double, 4> stage { { 0.0, 0.0, 0.0, 0.0 } };
+
+    // Comb delay line.
+    std::vector<float> combLine;
+    int    combWrite = 0;
+    double combDamp  = 0.0;
+    bool   combDirty = false;   // set once comb() has written to combLine
+
+    // Formant: three parallel bandpass SVFs (state pairs).
+    std::array<std::array<double, 2>, 3> formantState { { { { 0.0, 0.0 } }, { { 0.0, 0.0 } }, { { 0.0, 0.0 } } } };
+    std::array<std::array<double, 3>, 3> formantCoefficients {};
+
+    void   updateCoefficients (float cutoffHz, float res, int type) noexcept;
+    void   updateSvf (double g, double k) noexcept;
+    double svfBand  (double in, double g, double k) noexcept;
+    double ladder   (double in, double g, double res, double drive, int stages, bool diode) noexcept;
+    double comb     (double in, double freqHz, double res, double drive) noexcept;
+    double formant  (double in) noexcept;
+    double screamer (double in, double drive) noexcept;
+};
+
+} // namespace serum