feat(simulation): integrate knowledge graph into observer snapshot (#366)

VisibleEntity now carries relationship state (D-033 entity color) and
observation type (Visible vs Remembered). compute_observer_snapshot
queries the player's KnowledgeGraph to overlay relationship data on
visible entities and include remembered (not-in-LOS) entities as fog
ghosts at their last known position. Regenerated msgpack fixtures.

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

server/src/knowledge/types.rs

@@ -205,7 +205,7 @@ impl Default for DecayThresholds {
 
 /// How an entity appears in the observer snapshot.
 /// Extends VisibleEntity for knowledge-based rendering.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum EntityVisibility {
     /// Currently in line of sight.
     Visible,
@@ -215,3 +215,9 @@ pub enum EntityVisibility {
         age_ticks: u64,
     },
 }
+
+impl Default for EntityVisibility {
+    fn default() -> Self {
+        Self::Visible
+    }
+}