Summarize snap indices by actor category

Usage

snap_category_summary(fit, groups, years = NULL, side = c("u", "v"))

fit: a lame fit with snap-shift MCMC output, or a lame_snap_als fit. MCMC fits should be run with retained snap draws for posterior uncertainty; ALS fits contribute their snap_prob scores as one score row.
groups: a named list mapping group names to actor names/indices, or a named vector whose values are group labels and whose names are actors.
years: optional year/period names or indices. Default uses every period.
side: "u" for sender/row-side snap indicators or "v" for receiver/column-side indicators.

A data frame with one row per group-year.