Production Hourglass.Client.Backend implementation. Wraps the
Bridge NIFs to round-trip real Temporal Server.
Each callback opens its own bridge client lazily — matches the
pre-existing per-call connect pattern (Hourglass.Client.connect/1
did Client.connect() then Client.start_workflow(client, ...) on
every call). The cost is negligible compared to the gRPC round-trip
the call itself does.