PrefixSet: git-backed CIDR files via gitshallow, refreshed every 47 min.
DomainSet: takes pre-parsed (staticPrefixes, domains []string); resolves
hostnames every 5 min, retaining stale IPs on failure. Both use
atomic-swapped ipcohort.Cohort for lock-free reads.
ParseDomainSet(RecordReader) parses rows into prefixes and hostnames;
caller constructs the record reader (e.g. csv.Reader with delimiter,
comment char). RecordReader interface is satisfied by *csv.Reader.
commaify uses plain string concat (integers max ~7 groups).
