App Router: route groups prefetch cache should isolate parallel slots

[pedrolucas167]

Link to the code that reproduces this issue

https://github.com/pedrolucas167/next.js/tree/fix/app-router-route-groups-prefetch-cache

To Reproduce

1. Open the linked repository and run the App Router route-group scenario used by the regression fix.
2. Prefetch a route in one parallel slot / route group.
3. Navigate away and use browser back/forward so the route cache is consulted.
4. Observe whether the restored tree is isolated per parallel slot.

Current vs. Expected behavior

• Current: route cache entries can be shared across route-group / parallel-slot contexts.
• Expected: route cache entries should be isolated per parallel slot so back/forward restores the correct tree.

Additional context

This issue is related to a parallelSlot fix in the route cache key path. I’m linking my correction branch here for traceability.