Rename net/ipgate to net/ippolicy and compose independently refreshed
whitelist and blacklist sources into one IP decision policy.
- rename slog group and error prefixes from ipgate to ippolicy
- delete unused ParseDomainSet and RecordReader from domainsetcsv.go
- rewrite README to match the actual Config API
- document nil-whitelist behavior on New()
- bump iplist dependency to v0.8.5
- go mod tidy: remove stale ipgate and iplist entries
