Implement compute_nearby_interactions system that detects entities within close (≤2) and mid (≤5) Manhattan distance, computes available verbs per D-060 spec. NPCs get Talk+Observe at close range, Observe-only at mid range; PersonOfInterest flips priority. Objects get Examine. Results populate nearby_interactions[] on ObserverSnapshot v4. Bump protocol version 3→4. Implements #404. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>