Constant PROGRESS_QUEUE_HARD_LIMIT 

pub const PROGRESS_QUEUE_HARD_LIMIT: usize = 10_000;

Hard ceiling on the in-flight (progress) set. Reaching it means backpressure (crate::config::DispatcherConfig::max_in_flight) failed to hold the line, so we fail fast (panic → process abort → external restart) rather than exhaust memory — the dispatcher’s intentional fail-fast design. Backpressure must engage below this (asserted in tests).