Samoan choreographer Lemi Ponifasio calls his “Requiem” a ceremony. “It’s an act of preparation,” he said recently, on the phone from New Zealand. “It honors those who came before us and those who will join us. Mozart considered his ‘Requiem’ a conversation with God. I think of mine the same way.” A highlight of the Mostly Mozart Festival, the evening-length work, which will be performed at the Rose Theater on August 8 and 9, deals with themes similar to Mozart’s masterpiece: transformation, hope, and respect for life.