Initialize MixerEngine on startup (BB-6)

Add MixerEngine creation and wiring after model.Load() in main.cpp.
The mixer engine now gets created with current audio settings (sample rate,
buffer size from JackServerSettings), activated, and attached to the model.
This enables WebSocket mixer commands (mixerGetState, mixerAddChannel,
mixerAddBus, etc.) to work through /pipedal endpoint.

Missing initialization was the root cause of 'Invalid format' errors when
sending mixer commands via WebSocket.

src/main.cpp

@@ -29,6 +29,7 @@
 #include "WebServerConfig.hpp"
 #include <execinfo.h>
 #include "PiPedalSocket.hpp"
+#include "MixerEngine.hpp"
 #include "PluginHost.hpp"
 #include <boost/system/error_code.hpp>
 #include <filesystem>
@@ -451,6 +452,18 @@ int main(int argc, char *argv[])
 
             model.Load();
 
+            // Initialize Band-in-a-Box mixer engine
+            {
+                auto mixerEngine = std::make_shared<MixerEngine>();
+                // Configure with current audio settings
+                auto jackSettings = model.GetJackServerSettings();
+                mixerEngine->setSampleRate((uint32_t)jackSettings.GetSampleRate());
+                mixerEngine->setMaxBufferSize((size_t)jackSettings.GetBufferSize());
+                mixerEngine->Activate();
+                model.SetMixerEngine(mixerEngine);
+                Lv2Log::info("MixerEngine initialized (BB-6)");
+            }
+
             auto pipedalSocketFactory = MakePiPedalSocketFactory(model);
 
             server->AddSocketFactory(pipedalSocketFactory);