feat: add NS TTL consistency and NS-target CNAME checks

Observe the NS RRset TTL from each parent server (ParentView.NSTTL) and
whether each NS target name is a CNAME alias (ChildNSView.CNAMETarget).
Two new rules judge the collected facts:

- delegation_ns_ttl_inconsistent: warns when parent servers disagree on
  the NS TTL, which indicates zone-data inconsistency between primaries.
- delegation_ns_is_cname: flags NS targets that are CNAME aliases as
  critical, per RFC 2181 §10.3 which forbids aliased NS names.

checker/types.go

@@ -32,6 +32,8 @@ type ParentView struct {
 	UDPNSError   string               `json:"udp_ns_error,omitempty"`
 	TCPNSError   string               `json:"tcp_ns_error,omitempty"`
 	NS           []string             `json:"ns,omitempty"`
+	NSTTLKnown   bool                 `json:"ns_ttl_known,omitempty"`
+	NSTTL        uint32               `json:"ns_ttl,omitempty"`
 	Glue         map[string][]string  `json:"glue,omitempty"`
 	DSQueryError string               `json:"ds_query_error,omitempty"`
 	DS           []DSRecord           `json:"ds,omitempty"`
@@ -40,6 +42,7 @@ type ParentView struct {
 
 type ChildNSView struct {
 	NSName       string             `json:"ns_name"`
+	CNAMETarget  string             `json:"cname_target,omitempty"`
 	ResolveError string             `json:"resolve_error,omitempty"`
 	Addresses    []ChildAddressView `json:"addresses,omitempty"`
 }