Get specific domain data across all snapshots.
Examples
if (FALSE) { # \dontrun{
study <- create_longitudinal_study("STUDY-001", participants = 50, sites = 5,
snapshots = 3, domains = c("AE", "LB"))
ae_timeline <- get_domain_timeline(study, "AE")
length(ae_timeline) # one entry per snapshot
} # }