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.cpp

@@ -0,0 +1,35 @@
+#include "SubOscillator.h"
+
+namespace serum
+{
+
+void SubOscillator::noteOn()
+{
+    phase = 0.0;
+}
+
+void SubOscillator::processAdd (double freqHz, int shape, float level, float& out) noexcept
+{
+    if (level <= 0.0f || freqHz <= 0.0)
+        return;
+
+    // The octave shift is baked into freqHz by the voice; here we just phase-accumulate.
+    const double inc = juce::MathConstants<double>::twoPi * freqHz / sr;
+    phase += inc;
+    phase -= std::floor (phase * (1.0 / juce::MathConstants<double>::twoPi)) * juce::MathConstants<double>::twoPi;
+
+    float s = 0.0f;
+    if (shape == (int) SubShape::Triangle)
+    {
+        const float p = (float) (phase * (1.0 / juce::MathConstants<double>::twoPi));
+        s = 1.0f - 4.0f * std::abs (p - 0.5f);
+    }
+    else
+    {
+        s = (float) std::sin (phase);
+    }
+
+    out += s * level;
+}
+
+} // namespace serum