Function pickNextTempCandidateForPromotion

pickNextTempCandidateForPromotion(
tempSlotStreams: { id: string; scheduledStartTime: string }[],
): { id: string; scheduledStartTime: string }
Picks the temp-slot livestream with the earliest scheduled start (next in line for main-key promotion).
Parameters
- tempSlotStreams: { id: string; scheduledStartTime: string }[]
  Armed temp-slot livestreams for one user.
Returns { id: string; scheduledStartTime: string }
The earliest candidate, or null when the list is empty.
- Defined in lib/livestreams/key-assignment.ts:35