The Canadian composer Ari Kinarthy always dreamed of scoring a film, but he never expected it to be a film about his own life. The new documentary “Ari’s Theme” unfolds his remarkable story as someone living with type two spinal muscular atrophy — a rare genetic condition that progressively weakens the muscles in the body. Ari talks to Tom about the music he made for the film and why he wants it to be his legacy.