Flip resolver forwarding for k8s zones to the in-cluster bind (#288)

## Why

Step 3 of 3 in the external-dns → in-cluster bind migration: the client-visible cutover. The `openforwarder` resolvers currently forward `k8s.syd1.au.unkin.net` to the legacy VM anycast `198.18.19.20` (a temporary measure — commit 7ee5dfb) and have NO forwarder at all for the reverse zone `200.18.198.in-addr.arpa`. Once external-dns publishes to the in-cluster `bind-externaldns` (PR 2), resolvers must read from it.

## Changes

- Repoints the `fwd-k8s-syd1-au-unkin-net` forwarder from `198.18.19.20` (legacy VM) to `198.18.200.8` (in-cluster `bind-externaldns` VIP).
- Adds `fwd-200-18-198-in-addr-arpa` forwarding `200.18.198.in-addr.arpa` → `198.18.200.8`, closing the reverse-zone gap so PTR lookups for the k8s LB range keep resolving after cutover. Modeled exactly on the existing forward-zone entries.
- Refreshes the header comment to describe the in-cluster upstream.

`kubectl kustomize apps/overlays/au-syd1/bind-internal` builds clean; both zones render with forwarder `198.18.200.8` and there is no residual `198.18.19.20`.

## Merge gate

- PR 2 (`benvin/externaldns-incluster`) merged, AND
- record parity confirmed between legacy and in-cluster for the forward zone. Spot-check (repeat for each name):
  ```
  for n in puppetca puppet puppetdb encapi pdbmux artifactapi consul; do
    echo "$n:"
    dig +short @198.18.19.20 A $n.k8s.syd1.au.unkin.net
    dig +short @198.18.200.8  A $n.k8s.syd1.au.unkin.net
  done
  # plus 2-3 PTRs in the reverse zone:
  dig +short @198.18.19.20 -x 198.18.200.8
  dig +short @198.18.200.8  -x 198.18.200.8
  ```
  A/PTR answers from `198.18.200.8` must match those from `198.18.19.20` before merging.

## Verification (after merge)

```
dig +short @198.18.200.7 A puppet.k8s.syd1.au.unkin.net   # resolvers VIP
dig +short @198.18.200.7 -x 198.18.200.8                  # reverse via resolvers
```
Resolution through the `bind-resolvers` VIP should now answer for both the forward and reverse k8s zones.

## Rollback

Revert this PR — the `fwd-k8s-syd1-au-unkin-net` forwarder returns to `198.18.19.20` and the reverse forward is removed. The legacy VM is untouched and remains authoritative until decommission.

Reviewed-on: #288
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/bind-internal/resolvers/forward-zones.yaml

@@ -1,12 +1,9 @@
 # Conditional forward zones, from the puppet openforwarder view.
-# Upstreams: unkin authoritative 198.18.200.6, consul 198.18.19.14.
-# k8s -> TEMPORARY: the existing external external-dns bind service anycast
-#   198.18.19.20 (puppet roles::infra::dns::externaldns, ausyd1nxvm2127 + slaves),
-#   NOT the in-cluster bind-externaldns 198.18.200.8. The external service still
-#   holds the working k8s.syd1.au.unkin.net records; the in-cluster one is not
-#   reliably serving them yet, so forwarding there returns NXDOMAIN (which
-#   currently breaks Gitea's CI webhook: it cannot resolve the k8s CI host).
-#   Revert to 198.18.200.8 once external-dns publishes to the in-cluster service.
+# Upstreams: unkin authoritative 198.18.200.6, consul 198.18.19.14,
+#   k8s 198.18.200.8 (in-cluster bind-externaldns VIP).
+# k8s -> in-cluster bind-externaldns 198.18.200.8 for both the forward zone
+#   k8s.syd1.au.unkin.net and the reverse zone 200.18.198.in-addr.arpa, which
+#   external-dns now publishes to (see the external-dns migration PRs).
 # (Zones that forwarded to 10.10.16.x were dropped; consul left as-is.)
 ---
 apiVersion: bind.unkin.net/v1alpha1
@@ -63,10 +60,23 @@ spec:
   type: forward
   catalog: false
   forwarders:
-    # TEMPORARY: existing external external-dns bind service anycast, which
-    # currently holds the k8s.syd1.au.unkin.net records. Revert to 198.18.200.8
-    # (in-cluster bind-externaldns) once external-dns publishes there.
-    - 198.18.19.20
+    - 198.18.200.8
+---
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindZone
+metadata:
+  name: fwd-200-18-198-in-addr-arpa
+  namespace: bind-internal
+spec:
+  clusterRef: bind-resolvers
+  viewRef: openforwarder
+  zoneName: 200.18.198.in-addr.arpa
+  type: forward
+  catalog: false
+  forwarders:
+    # Reverse zone for the k8s LB range, published by external-dns to the
+    # in-cluster bind-externaldns alongside k8s.syd1.au.unkin.net.
+    - 198.18.200.8
 ---
 apiVersion: bind.unkin.net/v1alpha1
 kind: BindZone