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

argocd/applicationsets/platform.yaml

@@ -14,6 +14,7 @@ spec:
       - path: apps/overlays/*/artifactapi
       - path: apps/overlays/*/bind-system
       - path: apps/overlays/*/bind-internal
+      - path: apps/overlays/*/bind-external
       - path: apps/overlays/*/age-api
       - path: apps/overlays/*/cattle-system
       - path: apps/overlays/*/cert-manager

argocd/projects/platform.yaml

@@ -25,6 +25,8 @@ spec:
       server: https://kubernetes.default.svc
     - namespace: 'bind-internal'
       server: https://kubernetes.default.svc
+    - namespace: 'bind-external'
+      server: https://kubernetes.default.svc
     - namespace: 'cert-manager'
       server: https://kubernetes.default.svc
     - namespace: 'certificates'