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/perception/observer/mod.rs

@@ -17,6 +17,7 @@ use crate::perception::cognitive_delay::CognitiveDelay;
 use crate::perception::query::{ActivePerceptionMode, VisibilityGeometry};
 use crate::perception::vision_cone::Facing;
 use crate::simulation::contraband::ScanEventBuffer;
+use crate::simulation::conversation::ConversationEventBuffer;
 use crate::simulation::dialogue::DialogueResponseBuffer;
 use crate::simulation::interaction::NearbyInteractionBuffer;
 use crate::simulation::inventory::{CarriedBy, InventorySlot, ItemName};
@@ -78,6 +79,7 @@ pub fn compute_observer_snapshot(
             Option<&CognitiveDelay>,
             Option<&mut DialogueResponseBuffer>,
             Option<&mut ScanEventBuffer>,
+            Option<&mut ConversationEventBuffer>,
         ),
         With<PlayerCharacter>,
     >,
@@ -105,6 +107,7 @@ pub fn compute_observer_snapshot(
         cognitive_delay_opt,
         mut dialogue_response_opt,
         mut scan_event_buffer_opt,
+        mut conversation_buffer_opt,
     )) = observer_query.single_mut()
     else {
         tracing::error!("compute_observer_snapshot: PlayerCharacter query failed");
@@ -191,6 +194,12 @@ pub fn compute_observer_snapshot(
         .map(|buf| buf.take())
         .unwrap_or_default();
 
+    // Drain NPC-to-NPC conversation events (#247, D-078)
+    let (conversation_events, conversation_ended) = conversation_buffer_opt
+        .as_mut()
+        .map(|buf| (buf.take_events(), buf.take_ended()))
+        .unwrap_or_default();
+
     // Collect sound events audible to the observer (D-038, #124).
     // Filter by D-018 range: only events the player can hear based on distance.
     let sound_events = if let Some(ref queue) = sound_queue {
@@ -251,6 +260,8 @@ pub fn compute_observer_snapshot(
         dialogue_response,
         blocked_entities,
         scan_events,
+        conversation_events,
+        conversation_ended,
         sound_events,
         rng_seed: sim_rng.as_deref().map(|r| r.seed()),
     });