Builds one chain segment (a run of elements that all execute on the same
thread). Call ChainBuilder::queue to close the current segment behind
a Queue and start a new one on its own worker thread.
Top-level pipeline: one or more sources (see PipelineBuilder, with
everything reachable from each source’s own src pads already linked)
plus the bus every source reports events on and the Clock every
crate::elements::Pacer in it shares.