Cast the narrator for this cut.

Choose the voice that narrates this trailer, or send voiceId: null to clear it (the cut then narrates in the cast canon's NARRATOR when the canon casts one). The narrator belongs to the TRAILER: it need not be a cast member and need not match the table read's narrator. Storing the choice costs nothing — the voice is spoken at render — and re-casting is cheap, because narration is a separate track mixed over the finished cut, so a change re-renders audio and never a frame. Requires Idempotency-Key. Scope: story:write.