SAN FRANCISCO — It was a dream come true, straight out of “Antiques Roadshow.” In 2000 Rick Norsigian, a painter in a school maintenance department, bought a box of photo negatives at a garage sale in Fresno, Calif., for $45. Last month, a decade later, he stood in a Beverly Hills art gallery to announce that a team of experts had concluded “beyond a reasonable doubt” that Ansel Adams had taken the pictures.