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/mod.rs

@@ -7,7 +7,6 @@ use bevy_ecs::schedule::IntoScheduleConfigs;
 pub mod chunk_streaming;
 // Phase sub-plugins (#843)
 pub mod contraband;
-pub mod conversation;
 pub mod dialogue;
 pub mod economy;
 pub mod economy_plugin;
@@ -25,6 +24,7 @@ pub mod modification;
 pub mod monologue;
 pub mod movement;
 pub mod movement_plugin;
+pub mod npc_components;
 pub mod npc_knowledge_transfer;
 pub mod path_follow;
 pub mod pathfinding;