Public result returned by Mutare.transform_string/2.
The transform still produces richer internal Mutare.Site records for the runner and reporters.
This DTO is intentionally smaller and stable: callers get the rendered metamutant, the public
mutant descriptions, and the next available id without depending on internal site fields.
Summary
Types
@type t() :: %Mutare.Transform.Result{ metamutant: String.t(), mutants: [Mutare.MutationSite.t()], next_id: pos_integer() }