refactor(dns): eagerly decode CNAME target into r.data, drop target field

Address review feedback on #189:

- Remove Resource.target: Resource.Decode expands CNAME RDATA in place
  into r.data (uncompressed wire format) while the full message is still
  available and updates header.Length, storing the name bytes exactly once.
- Add Resource.CNAMEView returning a length-bounded view of the expanded
  target; Message.WriteAnswers uses it.
- Rename matchesHost to ResourceHeader.pertainsTo.
- Make ResolveConfig.MaxCNAMEs explicit: drop the silent default in
  Client.StartResolve and let callers opt in (x/xnet).
- Reduce test suite to regression coverage only.

refs #189

x/xnet/stack-async.go

@@ -631,6 +631,7 @@ func (s *StackAsync) StartLookupIPType(host string, qtype dns.Type) error {
 		},
 		EnableRecursion: true,
 		MaxIPs:          uint16(len(s.addrbufnip)),
+		MaxCNAMEs:       8,
 	})
 	if err != nil {
 		return err