dns: @hnw alternate proposal (#193)

* fix(dns): resolve CNAME chains and prevent invalid IP parsing

- Add automated in-band CNAME chain resolution to extract final A/AAAA IP addresses.
- Replace `MaxResponseAnswers` with `MaxIPs` and `MaxCNAMEs` to explicitly bound resource decoding and prevent memory exhaustion.
- Bound CNAME chain traversal to prevent infinite loops from cyclic records.

* 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

* dns: simplify @hnw function and additional fixes

---------

Co-authored-by: Yoshio HANAWA <y@hnw.jp>

x/xnet/stack-async.go

@@ -650,8 +650,10 @@ func (s *StackAsync) StartLookupIPType(host string, qtype dns.Type) error {
 		Additional: []dns.Resource{
 			s.ednsopt,
 		},
-		EnableRecursion:    true,
-		MaxResponseAnswers: uint16(len(s.addrbufnip)),
+		EnableRecursion: true,
+		// Leave headroom above the address buffer for CNAME records, which
+		// occupy answer slots before the addresses they alias.
+		MaxResponseAnswers: uint16(len(s.addrbufnip)) + 8,
 	})
 	if err != nil {
 		return err