DANBURY — A Spring Street man was arrested Wednesday for failing to register as a sex offender, authorities said. Joseph Castagnoli, 68, recently moved to Connecticut and has been seen in Danbury and New Milford, but did not contact the state’s sex offender registry, police said. According to a New York sex offender registry, he was charged with first-degree sexual abuse in that state in 1990. Castagnoli was held on a $50,000 bond.