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/Resources/Resources.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <JuceHeader.h>
+
+namespace serum
+{
+
+// ===========================================================================
+// Central dark theme + static SVG assets (embedded as strings so they need no
+// binary-data build step; the same assets also ship under Source/Resources/).
+// ===========================================================================
+namespace theme
+{
+    inline const juce::Colour bg          { 0xff101117 };
+    inline const juce::Colour panel       { 0xff1a1c24 };
+    inline const juce::Colour panelRaised { 0xff23262f };
+    inline const juce::Colour outline     { 0xff2c2f39 };
+    inline const juce::Colour text        { 0xffe8eaf0 };
+    inline const juce::Colour textDim     { 0xff9095a2 };
+    inline const juce::Colour accent      { 0xff4fc3f7 };   // cyan
+    inline const juce::Colour accent2     { 0xffff6e9c };   // pink
+    inline const juce::Colour amber       { 0xffffb74d };
+    inline const juce::Colour green       { 0xff7ce38b };
+    inline const juce::Colour raveGlow    { 0xffff3d71 };
+}
+
+// SVG assets (logo + subtle background).
+juce::String getLogoSvg();
+juce::String getBackgroundSvg();
+std::unique_ptr<juce::Drawable> createLogoDrawable();
+std::unique_ptr<juce::Drawable> createBackgroundDrawable();
+
+// Small value-formatting helpers used by the GUI controls.
+juce::String formatPercent (float v);   // 0..1 -> "42 %"
+juce::String formatHz      (float v);   // 0..1 (cutoff) -> "1.2 kHz"
+juce::String formatSeconds (float v);   // 0..1 (env) -> "320 ms"
+juce::String formatSemis   (float v);   // 0..1 -> "-12 st"
+juce::String formatCents   (float v);   // 0..1 -> "+5 ct"
+juce::String formatPan     (float v);   // 0..1 -> "L50"
+juce::String formatInt     (float v, int maxValue);
+
+} // namespace serum