Constant DEFAULT_SPIN_POLL_INTERVAL 

pub const DEFAULT_SPIN_POLL_INTERVAL: Duration;

Default per-iteration budget handed to spin_once by the spin loops.

phase-427 — the value spin_blocking carried as a private POLL_INTERVAL const before SpinOptions::poll_interval made it a field. It is named here because two verbs (Executor::spin, Executor::spin_forever) now share it, and a second literal is how the two would drift apart.