varar-runner — the imperative shell shared by var test-runner adapters.
Oath discovery (the shared glob semantics), planning/running examples,
failure rendering, and the filesystem varar.lock.json baseline store for
drift. Contains no pipeline logic — it delegates to varar-core. Steps are
supplied by the caller (Rust compiles step files in; there is no dynamic
load_steps), as a Registry plus a context factory.