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/simulation/mod.rs

@@ -5,6 +5,7 @@ use bevy_app::prelude::*;
 use bevy_ecs::schedule::IntoScheduleConfigs;
 
 pub mod contraband;
+pub mod conversation;
 pub mod dialogue;
 pub mod input;
 pub mod interaction;
@@ -48,6 +49,9 @@ impl Plugin for SimulationPlugin {
                     contraband::check_contraband_scan
                         .after(movement::validate_movement)
                         .before(crate::perception::observer::compute_observer_snapshot),
+                    conversation::run_npc_conversations
+                        .after(movement::validate_movement)
+                        .before(sound::collect_sound_events),
                     sound::collect_sound_events
                         .after(movement::validate_movement)
                         .before(crate::perception::observer::compute_observer_snapshot),