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/Modulation/AGENTS.md

@@ -0,0 +1,31 @@
+# modulation module
+
+## Owned files
+
+- ModulationMatrix.h, ModulationMatrix.cpp
+- MacroControls.h, MacroControls.cpp
+- RAVEButton.h, RAVEButton.cpp, which define `RaveController`
+
+## Rules
+
+- Use `bool` return codes for bounded inserts (`addConnection`, `addAssignment`).
+- Require a cap on connection and assignment counts (`kMaxConnections`, `kMaxAssignments`).
+- Serialise and restore state through `juce::ValueTree` so presets survive save and load.
+- Never accept a `ModTarget::None` connection; reject it and return false.
+- RaveController applies a non-destructive boost to render settings. It does not write APVTS parameters or keep a restoration snapshot.
+- Keep the existing RAVEButton filenames unless a rename is explicitly in scope.
+
+## GUI/audio state
+
+Hold `Engine::getControlLock()` while reading or editing the control-side matrix,
+macro assignments, or LFO shapes. The audio thread tries that lock once per block
+and copies into reserved audio-side storage. If the lock is busy, it keeps the
+previous settings and continues rendering. Never iterate the control-side vectors
+from the render path.
+
+Master and FX-mix modulation use global sources directly. Per-voice sources for
+those destinations come from the newest active voice.
+
+If `addConnection` or `addAssignment` returns false, handle the failed insert.
+
+This file refines the repository-root AGENTS.md for the related sources.