Function process_encoder 

pub fn process_encoder<const B: usize, const E: usize, const A: usize>(
    state: &mut PresetState<B, E>,
    preset: &Preset<B, E, A>,
    enc_idx: usize,
    direction: EncoderDirection,
    steps: u8,
) -> EngineResult

Process an encoder change. Applies steps, returns MIDI + display event.