refactor(engine): add thread-safe control capture and sub-block MIDI

Introduce a CriticalSection controlLock separating control-thread
state (matrix, macros, controlLfos) from audio-thread snapshots
(audioMatrix, audioMacros, lfos). captureControls() copies under
lock at the start of each processBlock; the audio thread never
touches control state directly. Cache parameter values in an
unordered_map keyed by string_view to avoid per-sample APVTS lookups.

Prebuild wavetables in the constructor instead of lazy allocation.
Resize mixBuffer only in prepare(), not per block. Render MIDI
events at their sample positions using sub-block rendering with
renderUntil(), so notes start at the correct sub-sample offset.

Make RaveController non-destructive: replace APVTS mutation with a
static apply() that boosts the RenderContext and FX slots for the
current block only. Remove the snapshot/restore machinery.

Make currentProgram atomic. Validate state I/O: check XML tag,
clamp program index, sanitize LFO shape data, and use
beginChangeGesture/endChangeGesture for RAVE toggle. Reset
parameters to defaults before loading preset values.

Source/PluginProcessor.h

@@ -1,16 +1,16 @@
 #pragma once
 
 #include <JuceHeader.h>
+#include <atomic>
 #include "Params.h"
 #include "Engine.h"
-#include "RAVEButton.h"
 
 namespace serum
 {
 
 // ===========================================================================
 // SerumAlt — a wavetable synthesiser (VST3 / AU). Hosts the APVTS, the audio
-// engine, preset management and the RAVE controller.
+// engine, preset management and the RAVE toggle.
 // ===========================================================================
 class SerumAltAudioProcessor : public juce::AudioProcessor
 {
@@ -52,15 +52,14 @@ public:
     void loadFactoryPreset (int index);
     int  getNumFactoryPresets() const;
 
-    // Public DSP state (read/write from the GUI thread).
+    // Public control state (hold the engine control lock for matrix/macros/LFOs).
     juce::AudioProcessorValueTreeState parameters;
     Engine engine;
-    RaveController rave;
 
     static juce::AudioProcessorValueTreeState::ParameterLayout createParameterLayout();
 
 private:
-    int currentProgram = 0;
+    std::atomic<int> currentProgram { 0 };
 
     void restoreLfoShapesFromState (const juce::ValueTree& state);
     void saveLfoShapesToState (juce::ValueTree& state) const;