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/mod.rs

@@ -244,7 +244,6 @@ impl Plugin for BridgePlugin {
                     crate::simulation::monologue::trigger_event_monologue
                         .after(crate::simulation::monologue::process_sprint_anomaly_monologue)
                         .after(crate::simulation::sound::collect_sound_events)
-                        .after(crate::simulation::conversation::run_npc_conversations)
                         .after(crate::simulation::dialogue::process_walk_away),
                     crate::simulation::monologue::process_contradiction_monologue
                         .after(crate::simulation::monologue::trigger_event_monologue),