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.

CMakeLists.txt

@@ -19,24 +19,24 @@ endif()
 add_subdirectory("${JUCE_ROOT}" "${CMAKE_CURRENT_BINARY_DIR}/third_party/JUCE")
 
 set(SERUMALT_SOURCES
-    Source/PluginProcessor.cpp
-    Source/PluginEditor.cpp
-    Source/Params.cpp
-    Source/Wavetable.cpp
-    Source/Oscillator.cpp
-    Source/SubOscillator.cpp
-    Source/NoiseOscillator.cpp
-    Source/Filter.cpp
-    Source/FilterBank.cpp
-    Source/Envelope.cpp
-    Source/LFO.cpp
-    Source/ModulationMatrix.cpp
-    Source/FXProcessor.cpp
-    Source/MacroControls.cpp
-    Source/RAVEButton.cpp
-    Source/SynthVoice.cpp
-    Source/Engine.cpp
-    Source/Resources.cpp
+    Source/Plugin/PluginProcessor.cpp
+    Source/GUI/PluginEditor.cpp
+    Source/Params/Params.cpp
+    Source/Synth/Wavetable.cpp
+    Source/Synth/Oscillator.cpp
+    Source/Synth/SubOscillator.cpp
+    Source/Synth/NoiseOscillator.cpp
+    Source/Synth/Filter.cpp
+    Source/Synth/FilterBank.cpp
+    Source/Synth/Envelope.cpp
+    Source/Synth/LFO.cpp
+    Source/Modulation/ModulationMatrix.cpp
+    Source/EffectUnits/FXProcessor.cpp
+    Source/Modulation/MacroControls.cpp
+    Source/Modulation/RAVEButton.cpp
+    Source/Synth/SynthVoice.cpp
+    Source/Engine/Engine.cpp
+    Source/Resources/Resources.cpp
     Source/EffectUnits/Hyper.cpp
     Source/EffectUnits/Chorus.cpp
     Source/EffectUnits/Flanger.cpp
@@ -57,9 +57,25 @@ set(SERUMALT_SOURCES
     Source/GUI/Panel.cpp
     Source/Presets/FactoryPresets.cpp)
 
+# One include directory per module, so a source names a header without a path
+# prefix. No two headers in this tree share a basename.
+set(SERUMALT_INCLUDE_DIRS
+    Source
+    Source/EffectUnits
+    Source/Engine
+    Source/GUI
+    Source/Modulation
+    Source/Params
+    Source/Plugin
+    Source/Presets
+    Source/Resources
+    Source/Synth
+    Source/Tests)
+
 function(serumalt_configure_target target)
     juce_generate_juce_header(${target})
     target_sources(${target} PRIVATE ${SERUMALT_SOURCES})
+    target_include_directories(${target} PRIVATE ${SERUMALT_INCLUDE_DIRS})
     target_compile_definitions(${target}
         PUBLIC
             JUCE_WEB_BROWSER=0