PLAY IT THROUGH. Puts the composition on the user's stage and plays it from frame 0 through the real preview — the path a person watches, which inspect_composition does not take — from the first frame to the last. Buffering is fine; any error ends it. Answers within about 20 s: {status:"running", at, total} means call again with the same id to keep waiting (it continues the same run). Then {status:"done", verdict:"ok"|"problems", problems, warnings} or {status:"incomplete", reason} — hidden (ask the user to bring the tab to the front), interrupted, edited. A piece is not finished until this says ok. It takes over the stage while it runs, so tell_user first.