Structured description of the product staleness behind a selection result.
Attached to every selection; a degraded result is never produced without it.
Epoch fields are seconds since the J2000 epoch (2000-01-01 12:00:00 in the
product's time scale). staleness_s is requested - source and is never
negative. kind is the degradation path (:exact, :nearest_prior, or
:diurnal_shift); staleness_days is staleness_s / 86400 (the whole-day
offset for a diurnal shift).