Function wire_key 

pub fn wire_key(ns: NonZeroU64, base_group_id: u64) -> u64

Wire key for one KV block: the namespace fold of the block’s BASE dense group id — group_id(GroupKey::new(layer, block, 0, K)), injective across (layer, block) within a layout (layout identity rides in the ns, so cross-layout ids cannot alias). Same splitmix fold as the SSM tier’s PagingSnapshotStore::wire — bijective per namespace, so no birthday risk over the dense group-id keyspace.