fix: crash when mount listener fires after GestureDetector unmount (#4268)

## Description

`useMountReactions` registers a `MountRegistry` listener in `useEffect`.
That listener can fire after the owning `GestureDetector` has already
unmounted (e.g. a related gesture mounts while this detector is gone).
Calling `updateDetector` in that case updates a detached detector and
can crash (occasionally in `findNodeHandle`).

Guard with the existing `state.isMounted` flag — the same pattern
`attachHandlers` already uses for the microtask after unmount.

Credit to @hannojg for the original investigation/fix direction.

## Changes

- In `useMountReactions`, return early from the mount listener when
`!state.isMounted`.

## Why not `useLayoutEffect`?

An earlier version of this PR also moved the subscription to
`useIsomorphicLayoutEffect` (same phase as GestureDetector attach/drop).
We A/B tested `useLayoutEffect` vs stock `useEffect`, both with this
early return. Crash rates were equivalent; the `isMounted` check alone
is sufficient, so this PR keeps `useEffect`.