refactor(server): fix clippy warnings from Rust 1.93

- Use #[derive(Default)] + #[default] instead of manual Default impls
  for FacingDirection, KnowledgeState, RelationshipState, EntityVisibility
- Replace manual modulo check with .is_multiple_of()
- Collapse nested if in shadowcast symmetry check

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/knowledge/events.rs

@@ -100,7 +100,7 @@ pub fn decay_knowledge(
     mut knowledge_query: Query<&mut KnowledgeGraph>,
 ) {
     // Decay runs every 10 ticks (1 game-minute per D-031)
-    if time.tick % 10 != 0 {
+    if !time.tick.is_multiple_of(10) {
         return;
     }
     for mut kg in knowledge_query.iter_mut() {