feat: integrate MixerEngine into AudioHost real-time audio pipeline

When SetMixerEngine() is called with a MixerEngine instance, the
audio processing thread routes all device input/output channels
through the mixer (channel strips → buses → master) instead of
the legacy Lv2Pedalboard. Falls back when no mixer engine is set.

- Thread-safe via mutex + realtime raw pointer swap (same pattern
  as existing SetPedalboard)
- Preserves existing pedalboard processing — both modes coexist
- New ProcessLv2Pedalboard() path when realtimeActiveMixerEngine
  is non-null uses device buffers directly for multi-channel I/O

src/AudioHost.cpp

@@ -24,6 +24,7 @@
 #include <lv2/atom/atom.h>
 #include "SchedulerPriority.hpp"
 #include "AlsaSequencer.hpp"
+#include "MixerEngine.hpp"
 
 #include "Lv2Log.hpp"
 
@@ -535,6 +536,10 @@ private:
     std::vector<std::shared_ptr<Lv2Pedalboard>> activePedalboards; // pedalboards that have been sent to the audio queue.
     Lv2Pedalboard *realtimeActivePedalboard = nullptr;
 
+    // Band-in-a-Box Mixer Engine
+    std::shared_ptr<MixerEngine> currentMixerEngine;
+    MixerEngine *realtimeActiveMixerEngine = nullptr;
+
     uint32_t sampleRate = 0;
     uint64_t currentSample = 0;
 
@@ -1197,6 +1202,24 @@ private:
 
     PIPEDAL_NON_INLINE void ProcessLv2Pedalboard(size_t nframes)
     {
+        // Band-in-a-Box Mixer Engine takes priority over legacy pedalboard
+        MixerEngine *mixerEngine = this->realtimeActiveMixerEngine;
+        if (mixerEngine != nullptr)
+        {
+            // Route through mixer engine using device input/output buffers
+            auto &driver = this->audioDriver;
+            if (driver) {
+                mixerEngine->process(
+                    driver->DeviceInputBuffers().data(),
+                    (uint32_t)driver->DeviceInputBufferCount(),
+                    driver->DeviceOutputBuffers().data(),
+                    (uint32_t)driver->DeviceOutputBufferCount(),
+                    (uint32_t)nframes
+                );
+            }
+            return;
+        }
+
         Lv2Pedalboard *pedalboard = nullptr;
 
         std::vector<float *> *pInputBuffers;
@@ -1944,6 +1967,22 @@ public:
         }
     }
 
+    virtual void SetMixerEngine(const std::shared_ptr<MixerEngine> &mixerEngine)
+    {
+        std::lock_guard guard(mutex);
+        this->currentMixerEngine = mixerEngine;
+        if (active && mixerEngine)
+        {
+            // Activate the mixer engine and set it as the realtime processing target.
+            // The mixer engine takes over from the legacy pedalboard.
+            this->realtimeActiveMixerEngine = mixerEngine.get();
+        }
+        else if (!mixerEngine)
+        {
+            this->realtimeActiveMixerEngine = nullptr;
+        }
+    }
+
     virtual void SetBypass(uint64_t instanceId, bool enabled)
     {
         std::lock_guard guard(mutex);