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/Oscillator.h

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "Params.h"
+#include "Wavetable.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Per-oscillator parameters (snapshot read from the APVTS each block).
+// ===========================================================================
+struct OscParams
+{
+    bool  enabled   = true;
+    int   wave      = 0;
+    float wtPos     = 0.0f;
+    int   warp      = 0;
+    float warpAmt   = 0.0f;
+    int   coarse    = 0;
+    int   fine      = 0;
+    float level     = 1.0f;
+    float pan       = 0.0f;
+    int   unison    = 1;
+    float detune    = 0.0f;
+    float spread    = 0.0f;
+    float phase     = 0.0f;
+    float randPhase = 0.0f;
+};
+
+// ===========================================================================
+// A polyphonic oscillator with wavetable morphing, warp modes and unison
+// (1..16 sub-voices). Per-voice state (phase) lives here; params are read each
+// block from the OscParams snapshot.
+// ===========================================================================
+class Oscillator
+{
+public:
+    Oscillator() = default;
+
+    void prepare (double sampleRate)          { sr = sampleRate; reset(); }
+    void reset();
+
+    // Initialise a fresh note's unison sub-voices: phase offsets, detune, pan and gain.
+    void noteOn (const OscParams& p, juce::uint32 seed);
+    void setParams (const OscParams& p) noexcept;
+
+    // Accumulate this oscillator's contribution into outL/outR.
+    void processAdd (const Wavetable& wt, const OscParams& p, double freqHz,
+                     float& outL, float& outR) noexcept;
+
+    int  getActiveUnison() const noexcept { return activeUnison; }
+
+private:
+    struct SubVoice
+    {
+        double phase       = 0.0;
+        double detuneRatio = 1.0;
+        float  panL        = 0.70710678f;
+        float  panR        = 0.70710678f;
+        float  level       = 1.0f;
+    };
+
+    std::array<SubVoice, kMaxUnison> voices;
+    int    activeUnison = 1;
+    int    initializedUnison = 0;
+    bool   paramsValid = false;
+    OscParams cachedParams;
+    double sr           = 44100.0;
+    juce::Random rng;
+
+    static constexpr double kTwoPi = juce::MathConstants<double>::twoPi;
+
+    float warpPhase (float phase, const OscParams& p) const noexcept;
+    float warpSample (float sample, const OscParams& p) const noexcept;
+};
+
+} // namespace serum