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>

pkg/membership/server.go

@@ -81,6 +81,25 @@ type Server struct {
 	// (non-loopback) bind. See dev/0004d-dataplane-acl.md for the full rationale
 	// and the residual metadata exposure this does NOT close.
 	RequireEncryptedRooms bool
+
+	// Posture is the node's security posture, surfaced on /healthz so an operator
+	// or a peer can detect a node NOT running the homogeneous enforce+ACL+TLS
+	// posture a secure cluster requires (audit 0008 N1). It is set by the command;
+	// the zero value (all false) reflects an unsecured dev node.
+	Posture Posture
+}
+
+// Posture describes the security posture a membershipd node runs with. It is
+// non-secret operational metadata (booleans + the store backend name), published
+// on /healthz so a monitor can flag a cluster member that is not enforce+ACL+TLS
+// — the weak node that would let an unauthenticated peer harvest the cluster's
+// forwarded traffic (audit 0008 N1).
+type Posture struct {
+	Enforce bool   `json:"enforce"`
+	ACL     bool   `json:"acl"`
+	TLS     bool   `json:"tls"`
+	Cluster bool   `json:"cluster"`
+	Store   string `json:"store"` // "sqlite" | "kv"
 }
 
 // NewServer wires the membership store and blob store into an http.Handler. The
@@ -390,7 +409,7 @@ func (s *Server) verifyOwnerSig(roomID, by string, sig, canonical []byte) (Membe
 // ---- handlers -------------------------------------------------------------
 
 func (s *Server) handleHealth(w http.ResponseWriter, _ *http.Request) {
-	writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
+	writeJSON(w, http.StatusOK, map[string]any{"status": "ok", "posture": s.Posture})
 }
 
 func (s *Server) handleCreateRoom(w http.ResponseWriter, r *http.Request) {