feat(env): tokenize hardcoded dev LAN addresses into cluster-vars (R-ENV-02)

R-ENV-02 promises dev and prod differ ONLY by scoped cluster-vars overrides. In fact
dev LAN addresses were hardcoded into base manifests, so clusters/prod would have come
up ANNOUNCING DEV ADDRESSES (192.168.1.x) on the prod LAN (192.168.2.x).

New cluster-vars knobs (base default = dev literal, per the file's existing convention):
  K3S_API_HOST          platform/cilium/helmrelease.yaml (+ the values.yaml mirror)
  CILIUM_LB_POOL_START  platform-config/cilium/l2-ip-pool.yaml
  CILIUM_LB_POOL_STOP   "
  UNIFI_L4_VIP          apps/vms/unifi/50-service-l4.yaml
  CRAFTY_L4_VIP         platform/crafty/service-l4.yaml
  S3_BACKUP_ENDPOINT    platform/keycloak/postgres-cluster.yaml, apps/wave-0/{nextcloud,
                        semaphore}-db.yaml, and platform/velero/helmrelease.yaml

Also fixes GATEWAY_VIP's base default, which was still the RETIRED libvirt address
10.73.0.240, and adds the GATEWAY_VIP patch that clusters/prod was missing entirely.

clusters/base/apps-vms.yaml gains postBuild.substituteFrom (it was the one consuming
Kustomization without it, so a ${TOKEN} there would have shipped as a literal string).
The envsubst contract is documented there: ${VAR} substitutes and escapes as $${VAR};
$VAR and $(cmd) are left alone; ${ARR[0]} hard-errors. The three UOS_URL expansions in
the unifi cloud-init are escaped accordingly.

PROD ADDRESS PLAN (operator, 2026-08-31):
  nodes .10/.11/.12 · control-plane VIP .9 (Ansible-side) · GATEWAY_VIP .20
  Cilium LB pool .20-.30

CONTAINMENT INVARIANT, now documented at the pool and in the prod overlay: GATEWAY_VIP
MUST equal CILIUM_LB_POOL_START. The shared Gateway pins no address, so Cilium LB-IPAM
gives it the first free address of a matching pool block. An initial prod proposal put
.20 outside a .21-.30 pool; the Gateway would have come up on .21 while split-horizon
coredns-custom kept answering *.home.keithjackson.dev with .20 — an address nothing
announces — blackholing in-cluster OIDC hairpin traffic while reporting healthy. The
pool was widened to .20-.30. The control-plane VIP (.9) stays outside the pool so
kube-vip and LB-IPAM can never contend.

Values prod still cannot know stay INVALID `TBD-PROD-*` placeholders on purpose: base
defaults are dev values, so an omitted patch means prod silently inherits a dev address
— far worse than a loud failed apply.

DEV IS BYTE-IDENTICAL. Verified by kustomize build over 13 paths before/after, then
flux envsubst with the dev cluster-vars values applied to both sides: zero drift. The
only deltas are the intended additive keys in the overlay ConfigMap itself. No
unresolved ${TOKEN} survives. render-golden green (10 goldens); yamllint clean.

Serves: R-ENV-02. Task: T-7.1 (prerequisite). Gate: MERGE GATE: PASS.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/cilium/helmrelease.yaml

@@ -45,9 +45,12 @@ spec:
     # [parity] MUST equal Ansible: kubeProxyReplacement: true
     kubeProxyReplacement: true
     # [parity] MUST equal Ansible: k8sServiceHost/k8sServicePort = the k3s API endpoint
-    # (k3s_api_host/k3s_api_port). This is the primary control-plane node IP (192.168.1.50).
+    # (k3s_api_host/k3s_api_port). This is the primary control-plane node IP.
     # Setting these wrong breaks Cilium's connection to the API server on adopt.
-    k8sServiceHost: "192.168.1.50"
+    # ${K3S_API_HOST} (cluster-vars, R-ENV-02): dev 192.168.1.50 | prod set in clusters/prod.
+    # The `platform` Flux Kustomization substitutes it; the Ansible side is already parameterized
+    # from inventory (k3s_api_host), so the parity contract holds per-environment, not per-literal.
+    k8sServiceHost: "${K3S_API_HOST}"
     k8sServicePort: 6443
     # --- Gateway API (ingress; replaces Traefik) — docs/09, ADR-0002 ---
     # [parity] MUST equal Ansible: gatewayAPI.enabled: true