Orchestrator Task: fetches the programs for one block of channels and hands them back grouped by channel, so the guide can give each channel row its programs in one write.

A fetch of a block's whole time window, or of the part before what it holds, sends minEndDate (programs still airing at its start or later); a fetch of the part after sends minStartDate (programs starting at or after what the block holds). The server compares both inclusively, so an extension repeats the programs the block holds at its edge, and the guide drops them (Schedule.applyPrograms).