Function owners_per_domain 

pub fn owners_per_domain(
    owners: &[NodeId],
    nodes: &[(NodeId, Locality)],
    isolation_tier: usize,
) -> BTreeMap<String, u32>

Owner counts per failure domain at isolation_tier. The largest value over vnode_count is the blast radius — the share of state that goes Restoring if that one domain fails at once.