Function observer

packages/effect/src/internal/fiberRuntime.ts:3760–3763

(count: number)

            })
            const cleanup = entries.map((r, i) => {
              const observer = (count: number) => {
                counts[i] = count
                checkDone()
              }
              r.listeners.addObserver(observer)
              return () => r.listeners.removeObserver(observer)
            })