Compact summary of a df_forest() result: the same header shown by
print.forest_stats(), then one line per metric with its draw count n
and each covariate's levels in plot order. A covariate whose level repeats
its name (the reference panel) shows the name alone; reference copies
drawn in a covariate panel are marked *.
Usage
# S3 method for class 'forest_stats'
summary(object, ...)