WebSocket clients (React UI, scripts, tests)
        │
        ▼  ws://host:port
┌──────────────────────────────────────────┐
│  main.rs — WebSocket listener            │
│  ├─ ServerState (Arc<Mutex<>>)           │
│  │   ├─ StateStore (project model)       │
│  │   └─ EngineSession                    │
│  ├─ command_handler.rs — text frames     │
│  ├─ rt_frame.rs — binary RT broadcast    │
│  └─ broadcast channel → all clients      │
│                                          │
│  audio_engine.rs                         │
│  ├─ cpal audio I/O                       │
│  ├─ ring buffers ↔ engine                │
│  └─ meter polling (~30fps)               │
│                                          │
│  recording.rs — mic input → WAV          │
│  ffi_bridge.rs — VST3/AU plugin stub     │
│  link.rs — Ableton Link stub             │
└──────────────────────────────────────────┘

{ "eventId": "transport_play", "payload": {} }
{ "eventId": "set_track_volume", "payload": { "track": 0, "volume": 95 } }

cd rust
cargo run -p songbird-headless
# Listens on ws://localhost:<port>