Normalize Only/Prefer values (ip4→ipv4, ip6→ipv6) so downstream
comparisons match correctly. Fix asymmetric fallback where caller
network=ip6 fell through to IPv4 but ip4 didn't fall through to IPv6
— both branches now check hard constraints (server.Only || callerPref)
only, allowing soft Prefer to fall back. Fix data race on cache item.ts
by reading it inside RLock. Fix Store mutating caller's dns.Msg by
copying before TTL override. Add singleflight dedup, semaphore-bounded
async refresh, DNS rcode error checking, and bump EDNS0 UDPSize to
4096. Add 25 tests.
ginuerzh