refactor(server): decompose observer pipeline and fix interaction boundary

Extract visibility geometry into a separate system behind a
PerceptionQuery trait, enabling D-017 perception mode swapping.
Two-stage pipeline: compute_visibility_geometry writes to
VisibilityGeometry resource, compute_observer_snapshot reads it.

Remove KnowledgeGraph from compute_nearby_interactions (simulation
phase boundary violation). Verb availability stays in simulation;
POI-based priority adjustment moves to observer via
apply_poi_verb_priority helper.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/simulation/mod.rs

@@ -31,12 +31,9 @@ impl Plugin for SimulationPlugin {
                     pathfinding::compute_paths.after(input::process_player_input),
                     path_follow::follow_paths.after(pathfinding::compute_paths),
                     movement::validate_movement.after(path_follow::follow_paths),
-                    interaction::compute_nearby_interactions
-                        .after(movement::validate_movement),
                     path_follow::cleanup_path_blocked.after(movement::validate_movement),
                     time::advance_tick
-                        .after(path_follow::cleanup_path_blocked)
-                        .after(interaction::compute_nearby_interactions),
+                        .after(path_follow::cleanup_path_blocked),
                 ),
             );