Function process_input_event 

pub fn process_input_event(
    ev: InputEvent,
    ui: &mut UiState,
    ctx: &InputContext<'_>,
    out: &mut Vec<Action>,
)

Process one InputEvent. Mutates UiState; appends produced actions to out. Pure data — does no I/O. The router reads GameState (via ctx.current for prestige_available() / powerups.iter() and via ui::hands::occupied_at for misclick gating) but never mutates it; all mutation flows through the produced Actions and apply_action.