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/tests/serialization.rs

@@ -32,6 +32,8 @@ fn observer_snapshot_roundtrip() {
             z: 0,
             kind: EntityKind::Npc,
             visibility: VisibilitySector::Forward,
+            relationship: RelationshipState::Unknown,
+            observation: EntityVisibility::Visible,
         }],
     );
 
@@ -158,6 +160,8 @@ fn all_entity_kind_variants_roundtrip() {
             z: 0,
             kind,
             visibility: VisibilitySector::Forward,
+            relationship: RelationshipState::Unknown,
+            observation: EntityVisibility::Visible,
         };
         let snapshot = test_snapshot(0, vec![entity]);
         let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
@@ -190,6 +194,8 @@ fn snapshot_v2_fields_roundtrip() {
             z: 0,
             kind: EntityKind::Player,
             visibility: VisibilitySector::Forward,
+            relationship: RelationshipState::Unknown,
+            observation: EntityVisibility::Visible,
         }],
         visible_tiles: vec![
             VisibleTile {