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/RAVEButton.h

@@ -1,29 +1,22 @@
 #pragma once
 
 #include <JuceHeader.h>
-#include "Params.h"
 
 namespace serum
 {
 
+struct RenderContext;
+struct FxSlotParams;
+
 // ===========================================================================
-// RAVE — one-shot "make it huge" control. Toggling on snapshots the current
-// values of the affected parameters and pushes unison, width, drive, OTT and
-// reverb to their boosted settings; toggling off restores the snapshot.
+// RAVE — non-destructive "make it huge" control. While enabled, rendering
+// boosts unison, width, drive, OTT and reverb in the current block's snapshots;
+// the underlying parameters remain unchanged when toggling on or off.
 // ===========================================================================
 class RaveController
 {
 public:
-    void setEnabled (bool enabled, juce::AudioProcessorValueTreeState& apvts);
-    bool isEnabled() const noexcept { return enabled; }
-    void resetSnapshot();
-
-private:
-    bool enabled = false;
-    std::vector<std::pair<juce::String, float>> snapshot;
-
-    void snapshotParam (const juce::String& id, juce::AudioProcessorValueTreeState& apvts);
-    void setParam (const juce::String& id, float value, juce::AudioProcessorValueTreeState& apvts);
+    static void apply (RenderContext& context, FxSlotParams* slots, int numSlots) noexcept;
 };
 
 } // namespace serum