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/Modulation/RAVEButton.cpp

@@ -0,0 +1,38 @@
+#include "RAVEButton.h"
+#include "SynthVoice.h"
+#include "FXProcessor.h"
+
+namespace serum
+{
+
+void RaveController::apply (RenderContext& context, FxSlotParams* slots, int numSlots) noexcept
+{
+    // Boost the static rendering parameters.
+    context.oscA.unison = juce::jlimit (8, kMaxUnison, context.oscA.unison);
+    context.oscB.unison = juce::jlimit (8, kMaxUnison, context.oscB.unison);
+    context.oscA.spread = 1.0f;
+    context.oscB.spread = 1.0f;
+    context.oscA.detune = 1.0f;
+    context.oscB.detune = 0.7f;
+
+    // Apply drive boosts.
+    context.filters.f1Drive = 0.6f;
+    context.filters.f2Drive = 0.6f;
+
+    // Boost FX-specific params (Hyper intensity / Reverb mix).
+    if (slots == nullptr)
+        return;
+    for (int i = 0; i < juce::jlimit (0, kNumFxSlots, numSlots); ++i)
+    {
+        auto& slot = slots[i];
+        if (slot.type == (int) FxType::Hyper)
+            slot.p[0] = 1.0f;          // OTT intensity 100%
+        else if (slot.type == (int) FxType::Reverb)
+        {
+            const float mix = std::isfinite (slot.mix) ? slot.mix : 0.0f;
+            slot.mix = juce::jlimit (0.0f, 1.0f, mix + 0.4f);   // reverb send +6dB-ish
+        }
+    }
+}
+
+} // namespace serum