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/tests/serialization.rs

@@ -27,8 +27,7 @@ fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
-        conversation_events: vec![],
-        conversation_ended: vec![],
+
         follow_state: None,
         character_pressure: None,
         rng_seed: None,
@@ -293,8 +292,7 @@ fn snapshot_v2_fields_roundtrip() {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
-        conversation_events: vec![],
-        conversation_ended: vec![],
+
         follow_state: None,
         character_pressure: None,
         rng_seed: None,
@@ -405,8 +403,7 @@ fn all_facing_direction_variants_roundtrip() {
             blocked_entities: vec![],
             scan_events: vec![],
             sound_events: vec![],
-            conversation_events: vec![],
-            conversation_ended: vec![],
+
             follow_state: None,
             character_pressure: None,
             rng_seed: None,
@@ -1456,8 +1453,7 @@ fn serde_default_fields_fill_in_when_missing_from_wire() {
         "blocked_entities": [],
         "scan_events": [],
         "sound_events": [],
-        "conversation_events": [],
-        "conversation_ended": [],
+
         "follow_state": null,
         "rng_seed": null
     });