The user list is composed before anything moves onto the bus

Found by reading the live mesh's own notes before touching it, which is where this
was heading next.

Composing the bus's user list was gated on the controller already being on the new
bus. That cannot work: the server needs its user list *before* anything moves onto
it. Step 2 of the whole change is exactly that — the server stands in the mesh
carrying nothing, on its own ports, while every node stays where it is. Under the
old gating that step was impossible: the module would come up, find no accounts
file, and its entrypoint would wait for one the controller had decided not to write.

So the only question is whether this machine runs the module that asked for the
file. A mesh that never moves has written a user list nothing reads, costing a few
hundred bytes on one node. The reverse cost a step that could not be taken.

Pinned by a test over the records of a mesh mid-change: everything running, nothing
on the new bus, and a user list that contains the controller — because a file
without it is a bus its own writer cannot connect to.

internal/broker/users_test.go

@@ -201,3 +201,41 @@ func TestTheAccountsFileRefusesAUserWithNoPassword(t *testing.T) {
 		t.Fatal("a user with no password hash was written")
 	}
 }
+
+// **A user list is composed for a bus the mesh has not moved onto yet**, and that is the whole of
+// step 2 (novox/hq ADR 0116): the server stands in the mesh carrying nothing, on its own ports, while
+// every node is still on the bus it was on.
+//
+// Pinned because the first version of the composing step got it backwards — it wrote the list only
+// once the controller was already on the new bus, which is a step that cannot be taken: the module
+// comes up, finds no accounts file, and waits for one the controller had decided not to write.
+func TestAUserListIsComposedBeforeAnythingMovesOntoTheBus(t *testing.T) {
+	// Exactly the records of a mesh mid-change: everything running, nothing on the new bus.
+	users, err := Users(Records{
+		Nodes:    []string{"anchor"},
+		Assigned: map[string][]Declared{"anchor": {{Module: "nats"}}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	hashes := map[string]string{}
+	for _, u := range users {
+		hashes[u.Username()] = "$2a$11$" + strings.Repeat("x", 22)
+	}
+	filled, missing := WithPasswords(users, hashes)
+	if len(missing) != 0 {
+		t.Fatalf("users with no credential: %v", missing)
+	}
+	accounts, err := ComposeAccounts(filled)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// The controller's own user above all: a file without it is a bus its writer cannot connect to,
+	// which is what the server would be left holding the moment it starts.
+	if !strings.Contains(accounts, `user: "controller"`) {
+		t.Fatalf("the composed list does not contain the controller:\n%s", accounts)
+	}
+	if !strings.Contains(accounts, `user: "node.anchor"`) {
+		t.Errorf("the composed list does not contain the machine running the bus")
+	}
+}