feat(simulation): NPC-to-NPC conversation system (#247, D-078)

Implements server-authoritative per-word occlusion for NPC conversations.
ConversationEventBuffer drains into ObserverSnapshot each tick so the client
receives only words audible from the player's position. Updates test fixtures
to include the new conversation_events and conversation_ended fields.

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

server/tests/serialization.rs

@@ -27,6 +27,8 @@ fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
+        conversation_events: vec![],
+        conversation_ended: vec![],
         rng_seed: None,
     }
 }
@@ -259,6 +261,8 @@ fn snapshot_v2_fields_roundtrip() {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
+        conversation_events: vec![],
+        conversation_ended: vec![],
         rng_seed: None,
     };
 
@@ -355,6 +359,8 @@ fn all_facing_direction_variants_roundtrip() {
             blocked_entities: vec![],
             scan_events: vec![],
             sound_events: vec![],
+            conversation_events: vec![],
+            conversation_ended: vec![],
             rng_seed: None,
         };
         let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");