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/dialogue.rs

@@ -27,7 +27,6 @@ use crate::knowledge::types::{FactId, KnowledgeConfidence, KnowledgeSource, Stab
 use crate::knowledge::{EntityRegistry, KnowledgeGraph};
 use crate::npc::interaction::{InteractionEvent, InteractionEventKind, InteractionMemory};
 use crate::npc::relationships::{TrustEvent, TrustEventQueue};
-use crate::simulation::conversation::{display_label_for_role, NpcColorIndex, NpcName};
 use crate::simulation::knowledge_grant::KnowledgeGrant;
 use crate::simulation::line_pool::LinePoolIndexResource;
 use crate::simulation::line_pool::{
@@ -35,6 +34,7 @@ use crate::simulation::line_pool::{
 };
 use crate::simulation::monologue::{MonologueBuffer, MonologueState};
 use crate::simulation::movement::PlayerCharacter;
+use crate::simulation::npc_components::{display_label_for_role, NpcColorIndex, NpcName};
 use crate::simulation::rng::SimRng;
 use crate::simulation::time::SimulationTime;
 use crate::storyteller::EngagementRecord;