Matthew Zachary was a promising 21-year-old concert pianist and composer when a brain tumor cost him the use of his left hand. As he battled his cancer over the next 13 years, he created the I’m Too Young for This! Cancer Foundation, so named because his doctors had misdiagnosed him at first for just that reason. This weekend, long symptom-free and with control over his left hand restored, he gives a concert for the first time in more than a decade.