Last night at Carnegie Hall, the American Symphony Orchestra presented “Opus Posthumous,” a concert devoted to works that were not first performed until after the deaths of their composers. These included an opera overture by Franz Schubert to an opera never published or performed in his lifetime, Anton Bruckner’s Symphony No. 00 (a study symphony he composed but did not consider suitable for performance), and Antonin Dvorak’s Symphony No. 1, which was composed for entry