Initial extraction: C++ MixerEngine standalone daemon

Extracted from op-pedal mixer-engine branch. Core components:

- MixerEngine / MixerChannelStrip / MixerBus — cleaned of LV2/Pedalboard/IHost/MidiMapper deps
- MixerControlServer — Unix domain socket JSON control interface
- main.cpp — JACK audio I/O client with headless fallback
- CMake build — auto-fetches nlohmann/json, links JACK
- 9 core tests passing

Architecture: JACK ports → MixerEngine::process() → JACK ports,
controlled via Unix socket JSON commands.

include/mixer/Types.hpp

@@ -0,0 +1,90 @@
+// Copyright (c) 2026 Ourpad Network
+// See LICENSE file in the project root for full license text.
+
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace mixer {
+
+/// Types of buses in the mixer architecture.
+enum class MixerBusType {
+    Master,   // Main L/R output — end of signal chain
+    Subgroup, // Named subgroup (Drums, Guitars, Vocals...)
+    Aux,      // Aux send bus (monitor mix or FX send)
+    FxReturn, // Stereo return from a shared FX processor (reverb, delay)
+};
+
+/// Channel type classification for mixer channel strips.
+enum class MixerChannelType {
+    Instrument,   // guitar, bass, keys — expects amp modeling
+    Mic,          // vocal mic — expects compressor, EQ, reverb chain
+    Line,         // line-level input (backing tracks, synths, drum machines)
+    AuxReturn,    // return from external FX processor
+};
+
+/// Configuration for a single aux send on a channel strip.
+struct AuxSendConfig {
+    float level = -96.0f;      // dB, -96 ≈ -inf (effectively off)
+    bool preFader = false;     // true = pre-fader (monitor send), false = post-fader (FX send)
+    int64_t targetBusId = -1;  // bus ID this sends to
+
+    bool isActive() const { return level > -90.0f && targetBusId >= 0; }
+};
+
+/// Per-channel high-pass filter configuration.
+struct HpfConfig {
+    bool enabled = false;
+    float frequency = 80.0f;   // Hz
+};
+
+/// Routing entry: a source (channel or bus) feeds a target bus with a level.
+struct MixerRouteEntry {
+    enum SourceType {
+        SourceChannel,
+        SourceBus
+    };
+
+    SourceType sourceType;
+    int64_t sourceId;       // channel instanceId or bus ID
+    int64_t targetBusId;    // the bus being fed into
+    float level = 0.0f;     // dB
+};
+
+/// Describes a mapping from a mixer bus to physical output channels.
+struct MixerOutputRoute {
+    int64_t sourceBusId;      // Bus to route from
+    int sourceStartChannel;   // Starting channel on the bus (0=L, 1=R)
+    int targetStartChannel;   // Starting physical output channel
+    int channels;             // Number of consecutive channels to route (1 or 2 typically)
+};
+
+/// A complete snapshot of the mixer state for save/restore.
+struct MixerSnapshot {
+    struct ChannelState {
+        int channelIndex;
+        float volume;
+        float pan;
+        bool mute;
+        bool solo;
+        MixerChannelType channelType;
+        std::string label;
+        bool hpEnabled;
+        float hpFrequency;
+        std::vector<float> auxSendLevels; // indexed by aux bus index
+    };
+    struct BusState {
+        int64_t id;
+        std::string name;
+        MixerBusType type;
+        float volume;
+        bool mute;
+    };
+    std::vector<ChannelState> channels;
+    std::vector<BusState> buses;
+    std::vector<MixerRouteEntry> routes;
+};
+
+} // namespace mixer