Constant ME_SLOT_UNKNOWN 

const ME_SLOT_UNKNOWN: u64 = u64::MAX; // 18_446_744_073_709_551_615u64

me_slot value meaning “this node’s slot has not been learned yet”.

Cannot collide with a real slot, unlike 0, which is both a legitimate index and what AtomicU64::default() would yield – a derived Default would silently claim we sit in slot 0 and make every reply leg look like it terminates at its padding.