Added a Summary group generic method so that range(), min(), max(), sum(), and prod() work on numeric series. Previously these fell through to base R defaults and operated on the internal representation; e.g. range(natural1()) wrongly returned c(0, Inf) instead of c(1, Inf).
dsct_transform() now warns when the declared range is wider than the function’s image.
num_discretes() now returns 0 with a warning when to < from (an empty range), consistently across all series types, rather than erroring. This acts as a guard when dsct_transform() specifies a function range that is wider than the mapped domain.
