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.

README.md

@@ -27,28 +27,17 @@ working builds and hands-on listening over elaborate test infrastructure.
 
 ```
 Source/
-  PluginProcessor.{h,cpp}     APVTS, presets, state, RAVE/UI-scale glue
-  PluginEditor.{h,cpp}        Tabbed GUI, owned controls, scaling, previews
-  Params.{h,cpp}              Parameter IDs, indexed groups, enums, mappings
-  Wavetable.{h,cpp}           256×2048 tables, prebuilt by the engine
-  Oscillator.{h,cpp}          Morph/warp/unison oscillator
-  SubOscillator.{h,cpp}       Sine/triangle sub
-  NoiseOscillator.{h,cpp}     White/pink noise
-  Filter.{h,cpp}              Seven filter models and cached coefficients
-  FilterBank.{h,cpp}          Two slots and routing
-  Envelope.{h,cpp}            ADSR with curve
-  LFO.{h,cpp}                 Continuous phase, sync and shape editing
-  ModulationMatrix.{h,cpp}    Connections
-  FXProcessor.{h,cpp}         Independent effect state per rack slot
-  EffectUnits/                Hyper, Chorus, Flanger, Phaser, Distortion, EQ,
-                              Compressor, Delay, Reverb
-  MacroControls.{h,cpp}       Four assignable macros
-  RAVEButton.{h,cpp}          Render-time boost, without rewriting parameters
-  SynthVoice.{h,cpp}          Per-voice oscillators, filters and envelopes
-  Engine.{h,cpp}              32 voices, MIDI timing, control snapshots, FX
-  GUI/                        Controls and previews
-  Presets/FactoryPresets.cpp  Factory preset definitions
-  Resources/                  SVG assets
+  Engine/         Voice pool, MIDI timing, control snapshots, FX rack
+  Synth/          Wavetables, oscillators, filters, envelopes, LFOs, voice
+  Modulation/     Matrix, macros, RAVE
+  Params/         Parameter IDs, indexed groups, enums, mappings
+  Plugin/         AudioProcessor: APVTS, presets, state, RAVE/UI-scale glue
+  GUI/            Editor, controls and previews
+  EffectUnits/    FX rack and the nine effect units
+  Presets/        Factory preset definitions
+  Resources/      Theme, formatters and SVG assets
+  Tests/          Headless QA harness
+  ExactCompare.h  Shared exact float comparison helpers
 ```
 
 The engine splits rendering at MIDI event positions and into segments of at most