Function once

tests/unit/gateway-supervisor.test.ts:64–67

(event: string, callback: (...args: unknown[]) => void)

      const handlers = new Map<string, (...args: unknown[]) => void>();
      return {
        once(event: string, callback: (...args: unknown[]) => void) {
          handlers.set(event, callback);
          return this;
        },
        listen() {
          queueMicrotask(() => handlers.get('listening')?.());
          return this;