feat(simulation): add proximity detection and interaction verbs

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>

server/src/simulation/mod.rs

@@ -5,6 +5,7 @@ use bevy_app::prelude::*;
 use bevy_ecs::schedule::IntoScheduleConfigs;
 
 pub mod input;
+pub mod interaction;
 pub mod movement;
 pub mod path_follow;
 pub mod pathfinding;
@@ -22,6 +23,8 @@ impl Plugin for SimulationPlugin {
         app.init_resource::<time::SimulationTime>()
             .insert_resource(rng::SimRng::new(0))
             .init_resource::<input::InputQueue>()
+            .init_resource::<interaction::NearbyInteractionBuffer>()
+            .init_resource::<crate::knowledge::EntityRegistry>()
             .add_systems(
                 Update,
                 (
@@ -29,8 +32,12 @@ 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),
+                    time::advance_tick
+                        .after(path_follow::cleanup_path_blocked)
+                        .after(interaction::compute_nearby_interactions),
                 ),
             );