Reduces a check_na_timing() object to a per-group overview table: frame and
missing counts, the percentage missing, the number of gaps, and the longest
gap. This is the print-side mirror of anivis's as_plot_data() - a compact
table for
the console or a report, rather than for a geom.
Usage
# S3 method for class 'check_na_timing'
summary(object, ...)