Status: OUT
Nearby-friends correctly enforces bidirectional visibility with staleness rejection on both notification and query paths, but `update_location` iterates all friends with haversine for each (O(friends)) rather than using the spatial grid that `get_nearby_friends` uses — meaning notification throughput degrades linearly with social graph density while query performance does not.
Correctness properties (bidirectional visibility, staleness enforcement) are sound, but the asymmetric use of spatial indexing between the two paths creates a scalability gap that undermines the notification path
Depends on (SL): nearby-friends-bidirectional-visibility, nearby-friends-staleness-enforced-on-both-paths