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/NoiseOscillator.cpp

@@ -0,0 +1,36 @@
+#include "NoiseOscillator.h"
+
+namespace serum
+{
+
+void NoiseOscillator::reset()
+{
+    pinkB0 = pinkB1 = pinkB2 = 0.0f;
+}
+
+void NoiseOscillator::noteOn (juce::uint32 seed)
+{
+    rng.setSeed (seed);
+}
+
+void NoiseOscillator::processAdd (int type, float level, float& out) noexcept
+{
+    if (level <= 0.0f)
+        return;
+
+    const float white = rng.nextFloat() * 2.0f - 1.0f;
+
+    float s = white;
+    if (type == (int) NoiseType::Pink)
+    {
+        // Paul Kellet's economy pink-noise filter.
+        pinkB0 = 0.99765f * pinkB0 + white * 0.0990460f;
+        pinkB1 = 0.96300f * pinkB1 + white * 0.2965164f;
+        pinkB2 = 0.57000f * pinkB2 + white * 1.0526913f;
+        s = pinkB0 + pinkB1 + pinkB2 + white * 0.1848f;
+    }
+
+    out += s * level;
+}
+
+} // namespace serum