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/simulation/social_plugin.rs

@@ -1,8 +1,6 @@
-//! Social simulation plugin — NPC conversations, knowledge transfer, disclosure.
+//! Social simulation plugin — NPC knowledge transfer, disclosure, and social systems.
 //!
-//! All systems run in [`TickPhase::Simulation`]. Intra-phase ordering:
-//! - conversations → knowledge_transfer (transfer reads conversation results)
-//! - conversations → sound collection (sound reads conversation events)
+//! All systems run in [`TickPhase::Simulation`].
 
 use bevy_app::prelude::*;
 use bevy_ecs::schedule::IntoScheduleConfigs;
@@ -20,16 +18,11 @@ impl Plugin for SocialPlugin {
             .add_systems(
                 Update,
                 (
-                    super::conversation::run_npc_conversations,
-                    super::npc_knowledge_transfer::transfer_npc_knowledge
-                        .after(super::conversation::run_npc_conversations),
-                    super::sound::collect_sound_events
-                        .after(super::conversation::run_npc_conversations),
+                    super::npc_knowledge_transfer::transfer_npc_knowledge,
+                    super::sound::collect_sound_events,
                     // Voice enrichment (D-138) — rewrite NPC text with voiced variants.
                     // No-op when VoiceCacheResource is absent.
                     crate::voice::integration::voice_enrich_dialogue_response,
-                    crate::voice::integration::voice_enrich_conversation_events
-                        .after(super::conversation::run_npc_conversations),
                     // POI discovery reads visibility geometry (also Simulation phase)
                     super::poi_discovery::discover_pois,
                     // Follow state reads visibility geometry + movement