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/tests/bridge_ipc.rs

@@ -4,7 +4,7 @@ use settled_reach_server::bridge::framing::{read_framed, write_framed};
 use settled_reach_server::bridge::local::LocalBridge;
 use settled_reach_server::bridge::types::*;
 use settled_reach_server::bridge::SimBridge;
-use settled_reach_server::simulation::time::DayPhase;
+use settled_reach_server::simulation::time::{DayPhase, TickRate};
 use std::os::unix::net::UnixStream;
 use std::path::PathBuf;
 use std::thread;
@@ -33,13 +33,14 @@ fn snapshot_roundtrip_over_unix_socket() {
         let bridge = LocalBridge::accept(&server_path).expect("failed to accept");
 
         let snapshot = ObserverSnapshot {
-            version: 3,
+            version: 4,
             tick: 42,
             game_time: GameTime {
                 day: 0,
                 time_of_day: 0,
                 day_phase: DayPhase::Morning,
                 paused: false,
+                tick_rate: TickRate::Full,
             },
             player_facing: FacingDirection::North,
             entities: vec![VisibleEntity {
@@ -53,6 +54,7 @@ fn snapshot_roundtrip_over_unix_socket() {
                 observation: EntityVisibility::Visible,
             }],
             visible_tiles: vec![],
+            nearby_interactions: vec![],
         };
 
         bridge