Constant DEFAULT_INGRESS_BYTE_BUDGET 

pub const DEFAULT_INGRESS_BYTE_BUDGET: usize = _; // 268_435_456usize

Default cap on total in-flight ingress bytes the engine may hold across the ingress queue, slot table, and pending async completion buffers at any instant. 256 MiB. Wire and application boundaries try_charge against this cap before admitting a payload; overflow emits the appropriate BudgetExceeded InfraEvent and drops the offending bytes.