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/main.rs

@@ -161,7 +161,9 @@ fn main() {
     match settled_reach_server::knowledge::CultureResolver::open(&systems_db_path) {
         Ok(resolver) => {
             tracing::info!("Culture resolver opened: {:?}", systems_db_path);
-            app.insert_resource(settled_reach_server::knowledge::CultureResolverResource(resolver));
+            app.insert_resource(settled_reach_server::knowledge::CultureResolverResource(
+                resolver,
+            ));
         }
         Err(e) => {
             tracing::warn!(
@@ -343,8 +345,6 @@ fn send_panic_error(app: &App, panic_msg: &str) {
         blocked_entities: vec![],
         scan_events: vec![],
         sound_events: vec![],
-        conversation_events: vec![],
-        conversation_ended: vec![],
         follow_state: None,
         character_pressure: None,
         rng_seed: None,