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/src/bridge/text_renderer.rs

@@ -300,6 +300,8 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            conversation_events: vec![],
+            conversation_ended: vec![],
             sound_events: vec![],
             rng_seed: None,
         }
@@ -425,6 +427,8 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            conversation_events: vec![],
+            conversation_ended: vec![],
             sound_events: vec![],
             rng_seed: None,
         };