feat(simulation): integrate observer visibility query (#112)

Replace unfiltered generate_snapshot with compute_observer_snapshot
that combines shadowcasting + vision cone to send only visible
entities and tiles. Enforces information asymmetry (D-011): NPCs
behind walls or in the blind spot are excluded from the snapshot.

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

server/tests/game_loop.rs

@@ -56,9 +56,15 @@ fn player_moves_north_through_full_pipeline() {
         rmp_serde::from_slice(&response).expect("deserialize snapshot");
 
     // Snapshot captures state at end of tick 0 (before advance_tick increments to 1)
+    assert_eq!(snapshot.version, 2);
     assert_eq!(snapshot.tick, 0);
     assert_eq!(snapshot.entities.len(), 1);
 
+    // v2 fields populated
+    assert_eq!(snapshot.game_time.day, 0);
+    assert_eq!(snapshot.game_time.day_phase, settled_reach_server::simulation::time::DayPhase::Morning);
+    assert!(!snapshot.game_time.paused);
+
     let player_entity = &snapshot.entities[0];
     // Player started at (16, 16, 0), moved north (y-1) to (16, 15, 0)
     // Render coords: (16.5, 15.5, 0)
@@ -66,6 +72,7 @@ fn player_moves_north_through_full_pipeline() {
     assert_eq!(player_entity.y, 15.5);
     assert_eq!(player_entity.z, 0);
     assert!(matches!(player_entity.kind, EntityKind::Player));
+    assert!(matches!(player_entity.visibility, VisibilitySector::Forward));
 
     // Clean up
     drop(reader);