feat(embeddednats): UNIBUS_NATS_MONITOR flag decoupled from debug log

Add a dedicated UNIBUS_NATS_MONITOR=1 toggle that opens the embedded
nats-server monitoring HTTP endpoint (127.0.0.1:8222, loopback only) so a
local metrics scraper can read /varz, /connz and /jsz for server-level
metrics (msgs/s, connections, KV bucket msgs, RAFT leader per stream,
restarts).

Previously the monitoring endpoint was only reachable via UNIBUS_NATS_DEBUG=1,
which is coupled to the verbose nats-server debug log: enabling the endpoint
also wrote routes/RAFT/room subjects to journald in clear, which regresses the
hardened posture (issue 0007). The two concerns are now decoupled.

The toggle computation is extracted to a pure function
natsLogOpts(debugEnv, monitorEnv) (noLog, debug, trace, monitor): MONITOR=1
opens the endpoint while keeping the log quiet (NoLog true / Debug false). The
inverse coupling is preserved for backward compatibility (DEBUG still implies
MONITOR). The 127.0.0.1 bind stays hardcoded — the monitoring endpoint has no
auth and must never be reachable from the network.

Deploy wiring versioned: additive systemd drop-in
membershipd-cluster.service.d/nats-monitor.conf (Environment=UNIBUS_NATS_MONITOR=1)
plus a "NATS server metrics" section in the cluster README with the rolling
activation runbook (magnus -> homer -> datardos) gated on R3 reconvergence
(followers 2/2) between nodes.

Tests: pure decoupling table (monitor on => log NOT debug; debug => monitor;
default closed) + a real embedded server with MONITOR=1 asserting /varz answers
200 on loopback:8222, and a server without the flag with the endpoint closed.
100% additive: behavior is identical without the flag. Bump app.md 0.10.0 ->
0.11.0.

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

pkg/embeddednats/embeddednats.go

@@ -103,17 +103,38 @@ func StartHostAuth(storeDir, host string, port int, auth server.Authentication)
 	return StartServer(ServerConfig{StoreDir: storeDir, Host: host, Port: port, Auth: auth})
 }
 
+// natsLogOpts maps the two independent environment toggles to the embedded
+// nats-server logging and monitoring flags. It is a pure function (no I/O) so the
+// decoupling between the two toggles can be unit-tested directly.
+//
+//   - UNIBUS_NATS_DEBUG="1" enables the nats-server logger (route/RAFT/JetStream
+//     errors); "2" additionally enables protocol tracing. Off by default so the
+//     server stays silent (NoLog) and production behavior is unchanged.
+//   - UNIBUS_NATS_MONITOR="1" opens the monitoring HTTP endpoint (loopback only)
+//     for a local metrics scraper to read /varz, /connz and /jsz.
+//
+// The two are DECOUPLED on purpose: enabling the monitoring endpoint must NOT turn
+// on the verbose debug log, which would write room subjects and routing metadata
+// to journald in clear and regress the hardened posture (issue 0007). The reverse
+// coupling is kept for backward compatibility: debug mode still exposes the
+// monitoring endpoint as well (debug implies monitor), so existing debugging
+// workflows are unchanged.
+func natsLogOpts(debugEnv, monitorEnv string) (noLog, debug, trace, monitor bool) {
+	debug = debugEnv == "1" || debugEnv == "2"
+	trace = debugEnv == "2"
+	monitor = monitorEnv == "1" || debug
+	noLog = !debug
+	return noLog, debug, trace, monitor
+}
+
 // StartServer launches an embedded nats-server with JetStream from cfg. It
 // blocks until the server is ready to accept connections (up to 5s) and returns
 // the running server; the caller must Shutdown it.
 func StartServer(cfg ServerConfig) (*server.Server, error) {
-	// Diagnostic toggle: UNIBUS_NATS_DEBUG=1 enables the embedded nats-server's own
-	// logger (route/RAFT/JetStream errors), which is otherwise silenced. Off by
-	// default so production behavior is unchanged; only set it when debugging the
-	// cluster route layer.
-	debugLevel := os.Getenv("UNIBUS_NATS_DEBUG")
-	debugNATS := debugLevel == "1" || debugLevel == "2"
-	traceNATS := debugLevel == "2"
+	// Map the two independent env toggles to the nats-server logging + monitoring
+	// flags. See natsLogOpts for the decoupling rationale (issue 0007).
+	noLog, debugNATS, traceNATS, monitorNATS := natsLogOpts(
+		os.Getenv("UNIBUS_NATS_DEBUG"), os.Getenv("UNIBUS_NATS_MONITOR"))
 	opts := &server.Options{
 		JetStream:  true,
 		StoreDir:   cfg.StoreDir,
@@ -122,15 +143,17 @@ func StartServer(cfg ServerConfig) (*server.Server, error) {
 		ServerName: cfg.ServerName,
 		DontListen: false,
 		// Keep the embedded server quiet by default; the host app logs the URLs.
-		NoLog:   !debugNATS,
+		NoLog:   noLog,
 		Debug:   debugNATS,
 		Trace:   traceNATS,
 		Logtime: true,
 		NoSigs:  true,
 	}
-	if debugNATS {
-		// Expose the nats-server monitoring endpoint (loopback) so the operator can
-		// inspect /jsz, /routez, /varz while debugging the cluster meta-group.
+	if monitorNATS {
+		// Expose the nats-server monitoring endpoint on LOOPBACK ONLY (never public):
+		// the operator (or a local metrics scraper) inspects /varz, /connz, /jsz,
+		// /routez. The 127.0.0.1 bind is mandatory because this endpoint has no auth;
+		// it must stay unreachable from the network.
 		opts.HTTPHost = "127.0.0.1"
 		opts.HTTPPort = 8222
 	}