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>

dns/definitions.go

@@ -197,6 +197,8 @@ const (
 	TypeALL   Type = 255 // ALL
 )
 
+func (tp Type) IsIPAddr() bool { return tp == TypeA || tp == TypeAAAA }
+
 // A Class is a type of network.
 type Class uint16