Workspace member 8 of 8. Library + binary; mirrored slice-2's
rutster-tap-echo shape. Three stub modules (api_key, translator,
openai_client) filled in by Tasks 3-5. No behavioral code yet —
this task only locks the crate boundary + the new async-trait dep
(the Tool trait in Task 6 needs it for async-fns-in-trait-objects).
