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/test_world/mod.rs

@@ -614,8 +614,8 @@ pub fn setup_gauntlet(app: &mut App, archetype: crate::bridge::types::CharacterA
     // can respond to Talk using content from the YAML dialogue pools.
     {
         use crate::npc::Npc;
-        use crate::simulation::conversation::NpcColorIndex;
         use crate::simulation::dialogue::{CurrentMood, DialogueProfile};
+        use crate::simulation::npc_components::NpcColorIndex;
 
         // (location, role) pairs matching server/content/campaigns/.../dialogue/ YAML pools.
         // Cycling through these gives NPC variety across rooms.