The four Box auth flows. Build an auth::Auth and pass it to
client::Client::new. Split from the transport runtime so the
auth surface has a descriptive path (D-193).
The hand-written runtime the generated Box Rust SDK ships against
(TR-Rust.5). It implements the machine-readable runtime contract
(gantry-contract V1): generated code calls only these declarations, and
this crate supplies the behavior — a retrying async network layer (jittered
backoff, 401 refresh, Retry-After), auth-token threading, request builders,
and response accessors.