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/bridge/debug.rs

@@ -14,9 +14,9 @@ use crate::bridge::types::{
 };
 use crate::knowledge::EntityRegistry;
 use crate::npc::Npc;
-use crate::simulation::conversation::NpcName;
 use crate::simulation::economy::{EconSimResource, EconStateResource};
 use crate::simulation::movement::{PlayerCharacter, TilePosition, WalkabilityMap};
+use crate::simulation::npc_components::NpcName;
 use crate::simulation::tier::ActiveSim;
 use crate::simulation::time::SimulationTime;
 use crate::simulation::triangle::TriangleState;