feat(engine): retire D-078 overheard conversation system (#848)

Per R-012: delete conversation.rs, both overheard content files, and
remove all 6 wire-up points (social_plugin, bridge/types, monologue,
voice/integration). Protocol version 22 → 23. Scope confirmed by
#842 audit — npc/ and content/global/ untouched. Surviving NPC
components (NpcName, NpcColorIndex, NpcConversation) migrated to
simulation/npc_components.rs for use by D-080 knowledge propagation.
Also applies pre-existing cargo fmt debt (names.rs and 4 others).

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

server/src/perception/observer/mod.rs

@@ -17,7 +17,6 @@ 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::examine::ExamineResultBuffer;
 use crate::simulation::follow::FollowTarget;
@@ -84,7 +83,6 @@ pub fn compute_observer_snapshot(
             Option<&CognitiveDelay>,
             Option<&mut DialogueResponseBuffer>,
             Option<&mut ScanEventBuffer>,
-            Option<&mut ConversationEventBuffer>,
             Option<&FollowTarget>,
             Option<&mut ExamineResultBuffer>,
         ),
@@ -121,7 +119,6 @@ pub fn compute_observer_snapshot(
         cognitive_delay_opt,
         mut dialogue_response_opt,
         mut scan_event_buffer_opt,
-        mut conversation_buffer_opt,
         follow_target_opt,
         mut examine_result_buffer_opt,
     )) = observer_query.single_mut()
@@ -225,12 +222,6 @@ 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 {
@@ -476,8 +467,6 @@ pub fn compute_observer_snapshot(
         dialogue_response,
         blocked_entities,
         scan_events,
-        conversation_events,
-        conversation_ended,
         follow_state,
         character_pressure: pressure_query
             .iter()