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>

http/httpraw/kvbuffer.go

@@ -300,25 +300,7 @@ func (kvb *kvBuffer) getFoldIdx(key string) int {
 // EqualFoldASCII reports whether a and b are equal under ASCII case folding.
 // Unlike strings.EqualFold it does not fold non-ASCII runes, so no multi-byte
 // rune such as U+212A KELVIN SIGN can alias a header key.
-func EqualFoldASCII(a, b string) bool {
-	if len(a) != len(b) {
-		return false
-	}
-	const asciiCapDiff = 'a' - 'A'
-	for i := 0; i < len(a); i++ {
-		ca, cb := a[i], b[i]
-		if ca >= 'A' && ca <= 'Z' {
-			ca += asciiCapDiff
-		}
-		if cb >= 'A' && cb <= 'Z' {
-			cb += asciiCapDiff
-		}
-		if ca != cb {
-			return false
-		}
-	}
-	return true
-}
+func EqualFoldASCII(a, b string) bool { return internal.EqualFoldASCII(a, b) }
 
 // reserve ensures need free bytes are available in the buffer, growing it when
 // permitted. It accounts for the byte-0 reservation on an empty buffer (see

http/httpraw/multipart.go

@@ -253,20 +253,7 @@ func trimOWS(b []byte) []byte {
 	return b
 }
 
-// equalFold compares b to the ASCII lowercase key, case insensitively.
+// equalFold compares b to key, case insensitively.
 func equalFold(b []byte, key string) bool {
-	if len(b) != len(key) {
-		return false
-	}
-	const asciiCapDiff = 'a' - 'A'
-	for i := range b {
-		c := b[i]
-		if c >= 'A' && c <= 'Z' {
-			c += asciiCapDiff
-		}
-		if c != key[i] {
-			return false
-		}
-	}
-	return true
+	return EqualFoldASCII(b2s(b), key)
 }