//! Builder lifecycle: `build()` spawns the drain, a second `build()` is
//! rejected, and dropping the `Guard` joins the drain thread cleanly.
//!
//! This is an integration test (its own binary), so the process-global
//! `REGISTRY` OnceLock starts fresh -- `build()` can succeed exactly once here,
//! which unit tests in the shared lib-test process cannot guarantee.
use ;