Function catchup_exempt 

pub fn catchup_exempt(community_id: &str, target_epoch: u64) -> bool

The dissolved-gate exemption: a base rekey may advance a SEALED community’s epoch only while a migration pointer is held, the flip hasn’t happened, and the target epoch does not exceed the pointer’s publish epoch. Lets a stale member walk their held root forward to the one m was sealed under, without ever re-opening the seal for anything else. Any read error fails closed (no exemption).