fix(0006d): enforce homogeneous cluster posture + publish posture on /healthz (audit 0008 N1)

A cluster is only as secure as its weakest node: the data plane forwards every
subject between nodes, so one node running without enforced auth lets an
unauthenticated peer Subscribe(">") on it and harvest the traffic forwarded from
the ACL'd nodes.

- validateClusterConfig now takes the auth mode and REFUSES to join a cluster
  unless --bus-auth enforce, regardless of bind (a clustered node is a production
  node; there is no safe dev cluster without auth). This binary therefore cannot
  BE the weak node.
- Server.Posture {enforce,acl,tls,cluster,store} is published on /healthz (non
  secret operational metadata, probe stays unauthenticated) so a monitor or peer
  can detect a cluster member not running enforce+ACL+TLS — covering a peer that
  runs a tampered/old binary outside this node's control.

Tests:
- TestAttack0008_N1: a clustered node with --bus-auth off is refused; the same
  node with enforce + full route security is allowed.
- TestClusterConfigPolicy: extended with off/soft clustered cases (refused) and
  the mode parameter throughout.
- TestHealthExposesPosture: /healthz returns the posture booleans + store backend.

CGO_ENABLED=0 go build/vet/test green; govulncheck 0 reachable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/membershipd/main.go

@@ -97,7 +97,7 @@ func main() {
 	}
 	// Cluster route guard (issue 0003a): a public cluster needs a route secret
 	// and mutual route TLS, and the route-TLS flags are all-or-nothing.
-	if err := validateClusterConfig(*clusterName, *bind, *clusterUser, *clusterPass, *routeTLSCert, *routeTLSKey, *routeTLSCA); err != nil {
+	if err := validateClusterConfig(*clusterName, *bind, *clusterUser, *clusterPass, *routeTLSCert, *routeTLSKey, *routeTLSCA, authMode); err != nil {
 		log.Fatalf("%v", err)
 	}
 
@@ -274,6 +274,17 @@ func main() {
 		srv.RequireEncryptedRooms = true
 		log.Printf("cleartext rooms: DISABLED (public bind requires end-to-end encryption)")
 	}
+	// Publish this node's posture on /healthz so a monitor (or a peer) can detect a
+	// cluster member not running the homogeneous enforce+ACL+TLS posture (audit
+	// 0008 N1). enforce implies the per-subject ACL in this binary (they are wired
+	// together above).
+	srv.Posture = membership.Posture{
+		Enforce: enforce,
+		ACL:     enforce,
+		TLS:     *tlsCert != "",
+		Cluster: clustered,
+		Store:   *storeBackend,
+	}
 
 	// Replicated anti-replay (issue 0006a, audit 0008 N3): a clustered node MUST
 	// share its nonce store across the cluster, or a request accepted on one node