fix: crash when mount listener fires after GestureDetector unmount

[kosmydel]

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.

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