feat: add WebsSocket API for mixer engine control

New files:
- MixerApi.hpp/cpp: Model-level API bridging WebSocket messages to
  MixerEngine control (channel volume/pan/mute/solo/hpf, bus control,
  routing, state queries)
- 15 new WebSocket message handlers in PiPedalSocket.cpp for full
  mixer control surface

Integration:
- MixerEngine member added to PiPedalModel with Get/Set accessors
- SetMixerEngine propagates to AudioHost rt processing pipeline
- Socket handler auto-wires MixerEngine to MixerApi on connect

Messages implemented:
  mixerSetChannelVolume, mixerSetChannelPan, mixerSetChannelMute,
  mixerSetChannelSolo, mixerSetChannelLabel, mixerSetChannelHpf,
  mixerGetState, mixerAddChannel, mixerRemoveChannel,
  mixerSetBusVolume, mixerSetBusMute, mixerAddBus, mixerRemoveBus,
  mixerRouteChannelToBus

src/MixerApi.hpp

@@ -0,0 +1,108 @@
+// Copyright (c) 2026 Ourpad Network
+// See LICENSE file in the project root for full license text.
+
+#pragma once
+
+#include <string>
+#include <memory>
+#include <functional>
+
+namespace pipedal {
+
+class MixerEngine;
+
+/// Mixer API — bridges WebSocket messages to the MixerEngine.
+///
+/// Follows the existing PiPedalSocket pattern where handlers are registered
+/// via REGISTER_MESSAGE_HANDLER and dispatched by message name.
+///
+/// This class provides the model-level methods that the socket handlers call.
+class MixerApi {
+public:
+    MixerApi();
+    ~MixerApi();
+
+    /// Set the mixer engine this API talks to.
+    void setMixerEngine(MixerEngine* engine) { mixerEngine_ = engine; }
+    MixerEngine* mixerEngine() const { return mixerEngine_; }
+
+    /// --- Channel Control ---
+
+    /// Set channel volume in dB (-inf to +12)
+    void setChannelVolume(int channelIndex, float volumeDb);
+
+    /// Set channel pan (-1.0 left to +1.0 right)
+    void setChannelPan(int channelIndex, float pan);
+
+    /// Set channel mute state
+    void setChannelMute(int channelIndex, bool mute);
+
+    /// Set channel solo state
+    void setChannelSolo(int channelIndex, bool solo);
+
+    /// Set channel label
+    void setChannelLabel(int channelIndex, const std::string& label);
+
+    /// Set channel type (Instrument, Mic, Line)
+    void setChannelType(int channelIndex, const std::string& type);
+
+    /// Set channel HPF state
+    void setChannelHpf(int channelIndex, bool enabled, float frequency);
+
+    /// --- Channel Lifecycle ---
+
+    /// Add a new channel for the given physical input
+    int addChannel(int physicalInputIndex);
+
+    /// Remove a channel
+    void removeChannel(int channelIndex);
+
+    /// --- Bus Control ---
+
+    /// Set bus volume
+    void setBusVolume(int64_t busId, float volumeDb);
+
+    /// Set bus mute
+    void setBusMute(int64_t busId, bool mute);
+
+    /// Add a new bus
+    int64_t addBus(const std::string& type, const std::string& name, int channels);
+
+    /// Remove a bus
+    void removeBus(int64_t busId);
+
+    /// --- Routing ---
+
+    /// Route a channel to a bus
+    void routeChannelToBus(int channelIndex, int64_t busId, float levelDb);
+
+    /// Route a bus to another bus
+    void routeBusToBus(int64_t sourceBusId, int64_t targetBusId, float levelDb);
+
+    /// Remove a route
+    void removeRoute(int64_t sourceId, int64_t targetBusId);
+
+    /// --- State Queries ---
+
+    /// Get the full mixer state as a JSON string
+    std::string getStateJson() const;
+
+    /// --- Scenes ---
+
+    /// Save current mixer state as a scene
+    std::string saveScene(const std::string& name);
+
+    /// Load a scene by name
+    bool loadScene(const std::string& sceneId);
+
+    /// List available scenes
+    std::string listScenes() const;
+
+    /// Delete a scene
+    bool deleteScene(const std::string& sceneId);
+
+private:
+    MixerEngine* mixerEngine_ = nullptr;
+};
+
+} // namespace pipedal