“If you excuse me — I can show you my deck,” Arthur says, darting upstairs. Moments later, he returns with a stack of Pokémon cards 5 inches high. “This is Code Breaker,” he explains. “It puts two of any card on top of your deck. It’s handy.” As he shuffles through the stack, explaining each card, it’s clear that he’s become something of an expert. At just 5 years old, Arthur has been battling neuroblastoma…