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/gen_fixtures.rs

@@ -38,6 +38,8 @@ fn fixture_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,
     }
 }
@@ -214,6 +216,8 @@ fn generate_msgpack_fixtures() {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
+        conversation_events: vec![],
+        conversation_ended: vec![],
         rng_seed: None,
     };
     write_fixture(