reflow_chain now takes the spread that owns the flow, defaulting to the active one,
because during loading there is not yet an active spread.PreviousTextFrame and NextTextFrame, and gives the story only to the frame at
the head of the chain. It is a single pass: an element's identifier is derived from its InDesign
one, so the frame that says it continues into another can record that link before the other frame
has been read.