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/serialization.rs

@@ -1,23 +1,25 @@
 //! IPC serialization round-trip tests (D-030 Layer 1: fixture-based).
 
 use settled_reach_server::bridge::types::*;
-use settled_reach_server::simulation::time::DayPhase;
+use settled_reach_server::simulation::time::{DayPhase, TickRate};
 use std::fs;
 
 /// Helper to create a minimal v2 snapshot for tests
 fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
     ObserverSnapshot {
-        version: 3,
+        version: 4,
         tick,
         game_time: GameTime {
             day: 0,
             time_of_day: 0,
             day_phase: DayPhase::Morning,
             paused: false,
+            tick_rate: TickRate::Full,
         },
         player_facing: FacingDirection::North,
         entities,
         visible_tiles: vec![],
+        nearby_interactions: vec![],
     }
 }
 
@@ -40,7 +42,7 @@ fn observer_snapshot_roundtrip() {
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
     let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");
 
-    assert_eq!(decoded.version, 3);
+    assert_eq!(decoded.version, 4);
     assert_eq!(decoded.tick, 42);
     assert_eq!(decoded.entities.len(), 1);
     assert_eq!(decoded.entities[0].entity_id, 1);
@@ -178,13 +180,14 @@ fn all_entity_kind_variants_roundtrip() {
 #[test]
 fn snapshot_v2_fields_roundtrip() {
     let snapshot = ObserverSnapshot {
-        version: 3,
+        version: 4,
         tick: 100,
         game_time: GameTime {
             day: 3,
             time_of_day: 720,
             day_phase: DayPhase::Evening,
             paused: true,
+            tick_rate: TickRate::Paused,
         },
         player_facing: FacingDirection::Southeast,
         entities: vec![VisibleEntity {
@@ -211,12 +214,13 @@ fn snapshot_v2_fields_roundtrip() {
                 visibility: VisibilitySector::Peripheral,
             },
         ],
+        nearby_interactions: vec![],
     };
 
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
     let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");
 
-    assert_eq!(decoded.version, 3);
+    assert_eq!(decoded.version, 4);
     assert_eq!(decoded.game_time.day, 3);
     assert_eq!(decoded.game_time.time_of_day, 720);
     assert_eq!(decoded.game_time.day_phase, DayPhase::Evening);
@@ -244,17 +248,19 @@ fn all_facing_direction_variants_roundtrip() {
 
     for dir in directions {
         let snapshot = ObserverSnapshot {
-            version: 3,
+            version: 4,
             tick: 0,
             game_time: GameTime {
                 day: 0,
                 time_of_day: 0,
                 day_phase: DayPhase::Morning,
                 paused: false,
+                tick_rate: TickRate::Full,
             },
             player_facing: dir,
             entities: vec![],
             visible_tiles: vec![],
+            nearby_interactions: vec![],
         };
         let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
         let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");