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/Plugin/AGENTS.md

@@ -0,0 +1,43 @@
+# plugin module
+
+## Owned files
+
+- PluginProcessor.h, PluginProcessor.cpp
+
+## Rules
+
+- Keep this the JUCE `AudioProcessor` boundary: APVTS, engine, RAVE, presets and state.
+- Declare the full parameter layout in `createParameterLayout()`.
+- Use `juce::ScopedNoDenormals` in `processBlock`.
+- Guard `getRawParameterValue` and `getParameter` results before dereferencing them.
+- Return silently on null or invalid state XML and invalid ValueTrees.
+- Use raw `new` only for JUCE-owned objects (`createEditor`, `createPluginFilter`).
+- Hold the engine control lock across preset and state changes; preserve parameter IDs and normalized values for saved-project compatibility.
+- Factory presets reset sound parameters before applying overrides, while preserving UI scale.
+- Save one copy of each custom state section. LFO shape arrays are stored as JSON text inside XML.
+- RAVE is an ordinary parameter that enables a render-time boost; it does not rewrite other parameters.
+
+## IF-THEN
+
+- IF state XML is null or invalid THEN return without touching the APVTS.
+- IF a parameter is missing from a preset load THEN skip it with an `if (auto* param = ...)` guard.
+
+## Examples
+
+```cpp
+// BAD: dereferences a possibly null XML result
+auto xml = getXmlFromBinary (data, size);
+auto state = juce::ValueTree::fromXml (*xml);
+```
+
+```cpp
+// GOOD: null guard, silent return
+std::unique_ptr<juce::XmlElement> xml (getXmlFromBinary (data, size));
+if (xml == nullptr)
+    return;
+juce::ValueTree state = juce::ValueTree::fromXml (*xml);
+if (! state.isValid())
+    return;
+```
+
+This file overrides /AGENTS.md where they conflict.