window (spec/04 §4.3): the rows whose reading of field lies in
[now − from, now − to), in order. A given to is exclusive, an absent to closes the
window at now inclusively, an absent from is open, so adjacent windows partition a
column. now is the context's, else the newest reading. With latest, each declared
identity's newest row of them (Macaw.Data.Window.latest_rows/3).