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/AGENTS.md

@@ -2,7 +2,7 @@
 
 ## Owned files
 
-- Related embedded assets and helpers: `../Resources.h`, `../Resources.cpp`
+- Resources.h, Resources.cpp
 - logo.svg
 - background.svg
 

Source/Resources/Resources.cpp

@@ -0,0 +1,94 @@
+#include "Resources.h"
+#include "Params.h"
+
+namespace serum
+{
+
+juce::String getLogoSvg()
+{
+    return R"svg(
+<svg xmlns="http://www.w3.org/2000/svg" width="180" height="40" viewBox="0 0 180 40">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0" stop-color="#4fc3f7"/>
+      <stop offset="1" stop-color="#ff6e9c"/>
+    </linearGradient>
+  </defs>
+  <path fill="url(#g)" d="M20 6 L34 6 L22 34 L8 34 Z"/>
+  <text x="42" y="27" font-family="Verdana, sans-serif" font-size="22" font-weight="bold" fill="#e8eaf0">SerumAlt</text>
+</svg>
+)svg";
+}
+
+juce::String getBackgroundSvg()
+{
+    return R"svg(
+<svg xmlns="http://www.w3.org/2000/svg" width="1200" height="720" viewBox="0 0 1200 720">
+  <rect width="1200" height="720" fill="#101117"/>
+  <radialGradient id="r" cx="0.5" cy="0" r="1.2">
+    <stop offset="0" stop-color="#1a1c24" stop-opacity="1"/>
+    <stop offset="1" stop-color="#101117" stop-opacity="0"/>
+  </radialGradient>
+  <rect width="1200" height="720" fill="url(#r)"/>
+</svg>
+)svg";
+}
+
+std::unique_ptr<juce::Drawable> createLogoDrawable()
+{
+    auto xml = juce::parseXML (getLogoSvg());
+    return xml != nullptr ? juce::Drawable::createFromSVG (*xml) : nullptr;
+}
+
+std::unique_ptr<juce::Drawable> createBackgroundDrawable()
+{
+    auto xml = juce::parseXML (getBackgroundSvg());
+    return xml != nullptr ? juce::Drawable::createFromSVG (*xml) : nullptr;
+}
+
+juce::String formatPercent (float v)
+{
+    return juce::String (juce::roundToInt (v * 100.0f)) + " %";
+}
+
+juce::String formatHz (float v)
+{
+    const float hz = maps::cutoffToHz (v);
+    if (hz >= 1000.0f)
+        return juce::String (hz / 1000.0f, 2) + " kHz";
+    return juce::String (juce::roundToInt (hz)) + " Hz";
+}
+
+juce::String formatSeconds (float v)
+{
+    const float s = maps::toSeconds (v);
+    if (s < 1.0f)
+        return juce::String (juce::roundToInt (s * 1000.0f)) + " ms";
+    return juce::String (s, 2) + " s";
+}
+
+juce::String formatSemis (float v)
+{
+    const int semi = (int) std::llround (v * 48.0f) - 24;
+    return juce::String (semi > 0 ? "+" : "") + juce::String (semi) + " st";
+}
+
+juce::String formatCents (float v)
+{
+    const int ct = (int) std::llround (v * 200.0f) - 100;
+    return juce::String (ct > 0 ? "+" : "") + juce::String (ct) + " ct";
+}
+
+juce::String formatPan (float v)
+{
+    const int p = (int) std::llround ((v - 0.5f) * 200.0f);
+    if (p == 0) return "C";
+    return (p < 0 ? "L" : "R") + juce::String (std::abs (p));
+}
+
+juce::String formatInt (float v, int maxValue)
+{
+    return juce::String (juce::jlimit (0, maxValue, (int) std::llround (v * maxValue)));
+}
+
+} // namespace serum

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