A repeat assignment says nothing changed (ADR 0115)

One assignment of a module per node is now the rule, not a limitation —
the operator dropped the multi-assignment requirement, and the schema's
(node, module) key has been the decision since migration 0005. What
changed: Assign reports whether the assignment was new, and the command
says 'already runs — one node runs one of each (ADR 0115); nothing
changed' instead of printing 'is assigned' for a no-op, which read as
an action that happened. Idempotence stays: a repeat is exit 0, because
a script stating what is already true is not wrong.

internal/inventory/adoption_test.go

@@ -65,7 +65,7 @@ func TestTakingIsRefusedOnAConvergedNodeAndForAnUnassignedModule(t *testing.T) {
 	if _, err := inv.AddNode(t.Context(), "converged"); err != nil {
 		t.Fatal(err)
 	}
-	if err := inv.Assign(t.Context(), "converged", "hello-web"); err != nil {
+	if _, err := inv.Assign(t.Context(), "converged", "hello-web"); err != nil {
 		t.Fatal(err)
 	}
 	if err := inv.Take(t.Context(), "converged", "hello-web"); !errors.Is(err, ErrNotAdopted) {
@@ -91,7 +91,7 @@ func TestATakenModuleOutlivesItsAssignmentAndReturningToAdopted(t *testing.T) {
 		t.Fatal(err)
 	}
 	for _, m := range []string{"hello-web", "postgres"} {
-		if err := inv.Assign(t.Context(), "anchor", m); err != nil {
+		if _, err := inv.Assign(t.Context(), "anchor", m); err != nil {
 			t.Fatal(err)
 		}
 	}