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/knowledge/culture.rs

@@ -92,7 +92,10 @@ pub struct CultureResolverResource(pub CultureResolver);
 /// - `CultureError::UnknownLocation` — not found in any table.
 /// - `CultureError::NoCulture`       — found but culture column is NULL (data bug).
 /// - `CultureError::Db`              — SQLite I/O failure.
-pub fn resolve_culture(resolver: &CultureResolver, location_id: &str) -> Result<CultureTag, CultureError> {
+pub fn resolve_culture(
+    resolver: &CultureResolver,
+    location_id: &str,
+) -> Result<CultureTag, CultureError> {
     let conn = resolver
         .conn
         .lock()
@@ -187,8 +190,8 @@ mod tests {
     use super::*;
 
     fn fixture_db() -> CultureResolver {
-        let path = Path::new(env!("CARGO_MANIFEST_DIR"))
-            .join("src/knowledge/fixtures/culture_test.db");
+        let path =
+            Path::new(env!("CARGO_MANIFEST_DIR")).join("src/knowledge/fixtures/culture_test.db");
         CultureResolver::open(&path).expect("open fixture DB")
     }
 
@@ -257,8 +260,8 @@ mod tests {
         use std::sync::Arc;
         use std::thread;
 
-        let path = Path::new(env!("CARGO_MANIFEST_DIR"))
-            .join("src/knowledge/fixtures/culture_test.db");
+        let path =
+            Path::new(env!("CARGO_MANIFEST_DIR")).join("src/knowledge/fixtures/culture_test.db");
         let r = Arc::new(CultureResolver::open(&path).expect("open"));
 
         let handles: Vec<_> = (0..4)