A built-by edge orders and gates a plan; it never widens it — and plans stop

The first live plan took seventy-five modules along for a controller change: the builder packages the
controller's source, everything is built by the builder, so everything was reachable. A module built by
the build machine is not changed by a new build machine. Reachability now follows the code and build
edges only; built-by still orders a tier after the build machine and gates it on the machine's roll-out.
plans stop <id> ends a plan by hand: what was asked still builds and registers, nothing further is asked.

cmd/mesh-controller/seatverbs.go

@@ -70,6 +70,9 @@ func argvFor(verb string, args map[string]any) ([]string, error) {
 		}
 		return []string{"builds"}, nil
 	case "plans":
+		if id := str("stop"); id != "" {
+			return []string{"plans", "stop", id}, nil
+		}
 		if id := str("id"); id != "" {
 			return []string{"plans", id}, nil
 		}