Add a primary (write) Service routing to pod-0

Secondaries reject RFC2136/nsupdate writes, but the read Service round-
robins across all pods. Add an optional per-cluster write endpoint that
targets only the primary pod (ordinal 0) via the StatefulSet pod-name
label. Reads keep using the all-pods Service.

- api: BindCluster.spec.primaryService (*ClusterServiceSpec) — ClusterIP
  for in-cluster writers (external-dns) or LoadBalancer for external
- reconcilePrimaryService creates <cluster>-primary selecting pod-0 when
  set, deletes it when unset
- regenerate CRDs + install.yaml

api/v1alpha1/bindcluster_types.go

@@ -105,10 +105,18 @@ type BindClusterSpec struct {
 	// +optional
 	Resources corev1.ResourceRequirements `json:"resources,omitempty"`
 
-	// Service controls how the cluster is exposed.
+	// Service controls how the cluster is exposed for reads (all pods).
 	// +optional
 	Service ClusterServiceSpec `json:"service,omitempty"`
 
+	// PrimaryService, when set, creates an additional Service routing only to the
+	// primary pod (ordinal 0) — the write endpoint for RFC2136/nsupdate, since
+	// secondaries reject updates. Reads still use Service (all pods). Use
+	// ClusterIP for in-cluster writers (e.g. external-dns) or LoadBalancer for
+	// external writers.
+	// +optional
+	PrimaryService *ClusterServiceSpec `json:"primaryService,omitempty"`
+
 	// NodeSelector for the BIND pods.
 	// +optional
 	NodeSelector map[string]string `json:"nodeSelector,omitempty"`

api/v1alpha1/zz_generated.deepcopy.go

@@ -301,6 +301,11 @@ func (in *BindClusterSpec) DeepCopyInto(out *BindClusterSpec) {
 	}
 	in.Resources.DeepCopyInto(&out.Resources)
 	in.Service.DeepCopyInto(&out.Service)
+	if in.PrimaryService != nil {
+		in, out := &in.PrimaryService, &out.PrimaryService
+		*out = new(ClusterServiceSpec)
+		(*in).DeepCopyInto(*out)
+	}
 	if in.NodeSelector != nil {
 		in, out := &in.NodeSelector, &out.NodeSelector
 		*out = make(map[string]string, len(*in))