Module engine 

The audio engine: owns the transport, synth, MIDI routing, and drives the audio callback.

Structs

Engine

Simple combined engine for tests and the TUI.

EngineAudio

Audio-thread-only state. NOT Send — lives inside the audio callback closure.

EngineShared

The core audio engine. Lives on the audio thread.

VuLevels

Shared VU meter levels — written by audio thread, read by UI thread. Stored as f32 bits in AtomicU32 (lock-free, no allocation).