Add extended forward zone type for overriding public domain records

An extended zone answers authoritatively only for the records it holds
locally; any name with nothing configured falls through to recursive
resolution instead of a synthesized NXDOMAIN, so a public domain's
records don't need to be duplicated just to override a few of them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8c76Xr2BycaHHu5N4buqi

internal/models/models.go

@@ -12,17 +12,29 @@ const (
 	ZoneForward  ZoneKind = "forward"
 	ZoneReverse4 ZoneKind = "reverse4"
 	ZoneReverse6 ZoneKind = "reverse6"
+	// ZoneExtended is a forward zone that is authoritative only for the
+	// records it explicitly holds. Any name it has no local data for falls
+	// through to recursive resolution instead of being answered NXDOMAIN, so
+	// it can be used to override a handful of records in an otherwise public
+	// domain without duplicating that domain's entire record set.
+	ZoneExtended ZoneKind = "extended"
 )
 
 // Valid reports whether k is a kind the application understands.
 func (k ZoneKind) Valid() bool {
 	switch k {
-	case ZoneForward, ZoneReverse4, ZoneReverse6:
+	case ZoneForward, ZoneReverse4, ZoneReverse6, ZoneExtended:
 		return true
 	}
 	return false
 }
 
+// IsForward reports whether k belongs on the forward-zone side of the UI
+// (as opposed to the reverse-zone side).
+func (k ZoneKind) IsForward() bool {
+	return k == ZoneForward || k == ZoneExtended
+}
+
 // Label returns a human readable name for the zone kind.
 func (k ZoneKind) Label() string {
 	switch k {
@@ -30,6 +42,8 @@ func (k ZoneKind) Label() string {
 		return "Reverse IPv4"
 	case ZoneReverse6:
 		return "Reverse IPv6"
+	case ZoneExtended:
+		return "Extended"
 	default:
 		return "Forward"
 	}