1.7 finished: minting, the file delivered, and a test flake I caused

**First, a correction: the previous commit went in on a false check.** Its message
says the suite passed; it did not. The check piped `go test` through a filter that
swallowed the failures and then printed "green" regardless. Two tests were failing
when 4de10e3 landed.

What was failing was my own doing. Purging the streams instead of deleting them
(4de10e3) left the *consumers* behind, because deleting a stream takes its
consumers with it and purging does not. A durable push consumer surviving between
tests keeps pushing to a delivery subject the previous test's subscription has gone
from: the messages count as delivered, go nowhere, and the next test waits out its
timeout for an announcement the server believes it already sent. Consumers are now
removed with the purge. Five consecutive clean runs.

`-p 1` stays, because two packages asserting and deleting the same fixed-name
objects on one bus is a real race — but its comment said the cause I had guessed
and not the one I found, so it now says the right thing.

**And delivery was not finished when I said it was.** Nothing filled
`Rendering.BusUsers`, so the composed file would never have reached a node.
`composeBusUsers` closes it: composed per push for the machine holding
`mesh-broker`, never kept, because the list is a function of the mesh's records and
a stored copy could disagree with them while both looked consistent. A user with no
credential is left out and named rather than written as a user without a password —
an ordinary situation with an obvious remedy — but a file with no users at all is
refused, because that bus would refuse every connection in the mesh.

**Minting, on both halves.** A node at enrolment and a module at `module issue`.
Three things differ from a management call and each is the point of the move: the
credential is minted into the mesh's records and becomes usable at the next
composition, so no server need be reachable; the password travels beside the address
rather than inside it, because a credential embedded in a URL leaks into every log
line that prints a connection; and a module's durable consumer is derived from what
it declared rather than named, so it cannot ask for delivery of something it did not
say it consumes.

A node reconnecting may be refused until that composition reaches the machine
running the bus. That is what the host's reconnect backoff is for and it is
survivable by design; waiting for the push would hold an enrolment open for as long
as a declaration takes to apply.

Tested that the switch is a switch: a node enrolling on one bus comes away with a
credential for that bus and none for the other, because one that held both could be
half-moved and nothing would say which half.

cmd/mesh-controller/modules.go

@@ -257,6 +257,21 @@ func moduleCommand(ctx context.Context, args []string) error {
 			return err
 		}
 
+		// Which bus this mesh is on. A module gets a credential for exactly one, and the two are
+		// made in entirely different ways: on the bus the mesh runs on today an account is a
+		// management call, and on the bus being built it is a row the next composition writes into
+		// the server's user list (novox/hq design 25 §4).
+		busAddress, onNATS, err := broker.OnNATS()
+		if err != nil {
+			return err
+		}
+		if err := broker.MustBeOneBus(os.Getenv(broker.AMQPVarName), busAddress); err != nil {
+			return err
+		}
+		if onNATS {
+			return issueOnTheNewBus(ctx, inv, m, *forNode, busAddress)
+		}
+
 		management, err := broker.ManagementFromEnvironment()
 		if err != nil {
 			return err
@@ -567,3 +582,77 @@ func mayIssue(m catalogue.Manifest) error {
 	}
 	return nil
 }
+
+// issueOnTheNewBus gives an assigned module its credential on the bus being built.
+//
+// **Three things differ from a management call, and each is the point of the move.** The credential
+// is minted into the mesh's records and becomes usable at the next composition, so there is no
+// server to be reachable for this to work. The password travels beside the address rather than inside
+// it, because the runtime's contract already separates them and a credential embedded in a URL is one
+// that leaks into every log line that prints a connection. And the module's durable consumer is
+// derived from what it declared rather than declared by name, so a module cannot ask for delivery of
+// something it did not say it consumes.
+func issueOnTheNewBus(ctx context.Context, inv *inventory.Inventory, m catalogue.Manifest,
+	node, busAddress string) error {
+
+	user := broker.Principal{Kind: broker.KindModule, Node: node, Module: m.Module}.Username()
+	password, err := inv.MintBusPassword(ctx, inventory.BusUser{
+		Username: user, Kind: inventory.BusModule, Node: node, Module: m.Module,
+	})
+	if err != nil {
+		return err
+	}
+
+	// Where the module is told to find the bus, and what certificate it must present. The same pair
+	// a node is told, for the same reason: a mesh's bus presents its own certificate, in no public
+	// trust store, so an address alone fails at TLS.
+	known, err := broker.FromEnvironment()
+	if err != nil {
+		return fmt.Errorf("cannot deliver a credential without knowing where the bus is: %w", err)
+	}
+	reachable, err := brokerReachableAt(ctx, inv, known, node)
+	if err != nil {
+		return err
+	}
+	held, err := json.Marshal(struct {
+		URL         string `json:"url"`
+		Fingerprint string `json:"fingerprint,omitempty"`
+		Node        string `json:"node"`
+		Module      string `json:"module"`
+		User        string `json:"user"`
+		Password    string `json:"password"`
+	}{
+		URL: "nats://" + reachable, Fingerprint: known.Fingerprint,
+		Node: node, Module: m.Module, User: user, Password: password,
+	})
+	if err != nil {
+		return err
+	}
+	if err := inv.AcceptSecretForModule(ctx, node, m.Module, "broker", string(held)); err != nil {
+		return err
+	}
+
+	// And how it hears what it consumes. Derived from its declaration, and only when it declared
+	// something: a module that consumes nothing needs no consumer, and creating one would be a
+	// durable subscription nobody reads.
+	if consumer, needed := broker.ConsumerFor(broker.Principal{
+		Kind: broker.KindModule, Node: node, Module: m.Module,
+		Emits: m.Emits, Consumes: m.Consumes, Serves: m.Tools,
+	}); needed {
+		js, err := broker.Dial(busAddress)
+		if err != nil {
+			return fmt.Errorf("the credential is minted and the mesh cannot reach the bus to create "+
+				"how %s hears what it consumes: %w", m.Module, err)
+		}
+		defer js.Close()
+		if err := js.EnsureConsumer(consumer); err != nil {
+			return err
+		}
+	}
+
+	fmt.Printf("bus user %s minted for %s, scoped to what it emits and consumes\n", user, m.Module)
+	fmt.Printf("  sealed to %s. It arrives with the next push — `push %s` to send it\n", node, node)
+	fmt.Printf("  and it works once the bus has been told: the user list is composed into the " +
+		"machine holding mesh-broker\n")
+	return nil
+}