A planned stratum: the rules and relations evaluated together at one stratum index.
Wraps the IR strata into a planner-friendly form that carries the actual
IR.Rule structs (not just their IDs) for the engine and explain_plan/1.
Summary
Types
@type t() :: %ExDatalog.Planner.Stratum{ index: non_neg_integer(), relations: [String.t()], rules: [ExDatalog.IR.Rule.t()] }