Replace 10 raw "ipv4"/"ipv6" string literals with preferIPv4/preferIPv6
constants, extract the duplicated fallback-condition check (repeated 4x
across resolve and lookupCache) into shouldReturn, and fix a misleading
doc comment on resolvePreference's default return.
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.
Fix async resolve goroutine using caller's cancelable context by
switching to context.WithoutCancel. Fix NewGRPCPlugin silently
returning broken resolver on connection failure. Fix "resolover"
typo in log field. Add doc comments to all exported symbols.
ginuerzh