Add bind-external namespace for externally-reachable zones

We self-delegate _acme-challenge.unkin.net into an acme.unkin.net zone we serve,
so cert-manager can solve Let's Encrypt DNS-01 over RFC2136/TSIG. That needs a
publicly-reachable authoritative BIND, separate from the internal estate.

- Add app bind-external (base + au-syd1 overlay) and register it in the platform
  ApplicationSet and AppProject destinations.
- Add BindCluster bind-external: authoritative-only, recursion off, no
  forwarding, transfers denied except the keyed catalog/zone AXFR; 2 replicas;
  primaryService is a dmz-pinned PureLB LoadBalancer (198.18.199.53).
- Add BindZone acme.unkin.net (primary, dynamicUpdate) and BindTSIGKey
  certmanager (hmac-sha256), whose Secret reflects into the cert-manager
  namespace for the rfc2136 solver.

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

apps/base/bind-external/zones.yaml

@@ -0,0 +1,19 @@
+---
+# Self-delegated ACME challenge zone. Google Cloud DNS holds a one-time
+# _acme-challenge.unkin.net CNAME -> _acme-challenge.acme.unkin.net and an
+# acme.unkin.net NS delegation pointing here; cert-manager writes the challenge
+# TXT records via RFC2136 authenticated with the certmanager key.
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindZone
+metadata:
+  name: acme-unkin-net
+  namespace: bind-external
+spec:
+  clusterRef: bind-external
+  zoneName: acme.unkin.net
+  type: primary
+  defaultTTL: 60
+  dynamicUpdate: true
+  updateKeyRef: certmanager
+  allowTransfer:
+    - key certmanager