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/tests/error_handling.rs

@@ -288,8 +288,7 @@ fn snapshot_with_sim_errors_roundtrips() {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
-        conversation_events: vec![],
-        conversation_ended: vec![],
+
         follow_state: None,
         character_pressure: None,
         rng_seed: None,