Which tests to run for one mutant, before command-line serialization.

:suite runs everything; {:files, paths} runs whole files; {:tests, paths, names} narrows those files to the named tests; and {:app, dirs} runs an umbrella app and its dependents' test directories. :no_coverage launches no run. File, directory, and name lists are nonempty: an empty attribution must be resolved explicitly to a broader selection, never passed along as an accidental whole-suite command.

The constructors enforce nonemptiness and sort for deterministic execution. Coverage summaries and result labels read shape/1; only Mutare.Sandbox.Command serializes a runnable selection to Mix arguments.