feat(ci): substitution-safety gate + escape 6 comment hazards; strict mode is now SAFE

Adds conformance/tests/l0-static/flux-postbuild-substitution/ — an offline, stdlib-only
L0 check tagged R-ENV-02 + R-GITOPS-01 (deliberately NOT R-TEST-01, which would
overclaim). It reuses lib/status.py's `resources:` walk rather than forking it, so "what a
path renders" has exactly one definition. It asserts: every ${...} on a substituted path
is defined-or-escaped (comments included); no default-operator form on an undefined name;
no unparseable expansion; the rendering path actually substitutes cluster-vars; every
overlay-patched /data/KEY has a base default; and that the gate itself is load-bearing.

WIRED AS AN UNCONDITIONAL STAGE (3c) in ci/l0.sh, and that matters: stage 3b scopes to
touched requirements, and ci/touched-requirements.sh maps only clusters/* to R-ENV-02 —
so an edit to a substituted manifest under apps/ or platform/ would have filtered this
check out of exactly the change that introduces the bug. l0 fail -> BLOCK=1 in gate.sh.

Mutation-tested all five assertion classes (undefined token, indexed expansion, :- form,
missing substituteFrom, un-wired gate); each fails as intended, baseline green.

STRICT MODE IS PROVEN SAFE TO ENABLE — measured, not inspected. For all 11 substituted
paths, `kubectl kustomize --load-restrictor LoadRestrictionsNone <path> | flux envsubst
--strict` exits 0 against cluster-vars base defaults. Negative control confirms the
incident: with K3S_API_HOST unset, non-strict renders k8sServiceHost empty while strict
exits 1 with `variable not set (strict mode)`. The feature gate itself is NOT flipped
here — that is a live-cluster change and the operator's call.

Escapes the six comment hazards that stood in the way (apps/vms/unifi x4,
platform/longhorn x2). All were prose ABOUT the escaping convention; since the repo's own
contract is that $${...} emits a literal ${...}, escaping them is both precise and
self-consistent. kustomize strips these particular comments today, but comment survival is
an implementation detail — a body comment carrying $ WAS observed surviving into built
apps/wave-1 — and a survivor would kill the whole Kustomization.

DOCTRINE CORRECTED (measured against flux 2.4.0): clusters/base/apps-vms.yaml claimed an
indexed expansion is fatal "escaped or not". Escaping is TOTAL — $${ARR[0]} emits the
literal text safely; only the UNESCAPED indexed form is fatal. Corrected in place.

Also empties DOCUMENTED_GAPS in conformance/run.sh with rationale: R-ENV-02 and
R-GITOPS-01 now have a real verifying test, which run.sh's own comment named as the
trigger for removing them.

Advisory output (not failures): 6 dead cluster-vars knobs — BASE_DOMAIN, CLUSTER_ENV,
CLUSTER_POD_CIDR, PIA_REGION, PLATFORM_NODE_SELECTOR_{KEY,VALUE}. The last four are
documented reference-only; BASE_DOMAIN and CLUSTER_ENV look like genuine rot (hostnames
are hardcoded elsewhere) and are worth a follow-up.

Gate: MERGE GATE: PASS. 102 assertions, 0 hazards.

Serves: R-ENV-02, R-GITOPS-01.

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

platform/longhorn/helmrelease.yaml

@@ -2,7 +2,7 @@
 # Longhorn — distributed replicated block storage across the 3 nodes (ADR-0003).
 # Serves R-STATE-01 (state survives destroy/rebuild) and R-STATE-02 (3-way replication).
 #
-# Dev-appropriate values. Environment differences enter ONLY via ${...} substitution from
+# Dev-appropriate values. Environment differences enter ONLY via $${...} substitution from
 # cluster-vars (R-ENV-02); everything else is identical base config for dev and prod.
 apiVersion: helm.toolkit.fluxcd.io/v2
 kind: HelmRelease

platform/longhorn/storageclasses.yaml

@@ -20,7 +20,7 @@ parameters:
   # (replicaSoftAntiAffinity=false) guarantees these land on distinct nodes, so any single node
   # loss keeps the volume available.
   # HARDCODED, not ${LONGHORN_DEFAULT_REPLICA_COUNT}: StorageClass.parameters is map[string]string,
-  # but kustomize strips the quotes around a bare "${VAR}" placeholder, so Flux postBuild substitution
+  # but kustomize strips the quotes around a bare "$${VAR}" placeholder, so Flux postBuild substitution
   # yields an unquoted numeric (3) that server-side apply rejects (int, not string). A quoted numeric
   # literal like "3"/"30" IS preserved by kustomize. The replica knob still drives the GLOBAL default
   # via defaultSettings.defaultReplicaCount in helmrelease.yaml (a Helm int value, unaffected).