a stale observer must not update the display

[Fil]

closes #174

todo:

• add test
• type Observer? like so:

  interface Observer {
    pending?(): void;
    fulfilled?(value: unknown): void;
    rejected?(error: unknown): void;
  }

[mbostock]

I don’t think this is the right fix, and I’m not sure that the linked issue is a bug. I’ll look more…

[Fil]

#174 (comment)