The bus is never public: the broker port is no longer a foundation opening

The controller widened the bus's from-mesh port to from-anywhere on the
broker's host so a machine could enrol before it had a tunnel. ADR 0169
has machines join through the tunnel and decides the bus is never public;
every live bus connection already comes from the mesh.

internal/catalogue/the_bus_is_never_public_test.go

@@ -0,0 +1,23 @@
+package catalogue
+
+import (
+	"regexp"
+	"testing"
+)
+
+// The bus is never public (novox/hq ADR 0169). Its port is what the bus module declares, the mesh,
+// and the control plane adds no opening of its own: a machine joins through the tunnel, so the
+// broker's host is filtered like any other. Before this, the broker port was a foundation port and
+// rendered from anywhere beside its from-the-mesh rule.
+func TestTheBusPortIsReachedFromTheMeshAlone(t *testing.T) {
+	rules := []Rule{{Port: 4222, Protocol: "tcp", From: FromMesh, Because: []string{"nats"},
+		Why: []string{"the mesh bus"}}}
+	out := AsNftables(rules, []string{"10.10.0.1", "10.10.0.2"}, true, nil, []string{"eth0"}, "mesh0")
+
+	if !regexp.MustCompile(`ip saddr \{ 10\.10\.0\.1, 10\.10\.0\.2 \} tcp dport 4222 accept`).MatchString(out) {
+		t.Fatalf("the bus is not reachable from the mesh:\n%s", out)
+	}
+	if regexp.MustCompile(`(?m)^\s*tcp dport 4222 accept`).MatchString(out) {
+		t.Fatalf("the bus is reachable from anywhere:\n%s", out)
+	}
+}