Function err_is_resume_at_capacity 

pub fn err_is_resume_at_capacity(err: &Error) -> bool

True when err is the typed ResumeAtCapacity backpressure marker (an over-cap resume was refused), as opposed to a store fault. Lets a caller in another module or crate (e.g. resolve_gate, or the gateway resume endpoint) render it as backpressure (HTTP 503) rather than a fault without depending on the private struct.