Function process_button 

pub fn process_button<const B: usize, const E: usize, const A: usize>(
    state: &mut PresetState<B, E>,
    preset: &Preset<B, E, A>,
    btn_idx: usize,
    event: ButtonEvent,
) -> EngineResult

Process a button event. Updates state and returns MIDI/system/display actions.