test(simulation): expand test coverage per PR #2 review

- Add roundtrip tests for all PlayerAction variants (Hoshe #6)
- Add roundtrip tests for all EntityKind variants (Hoshe #6)
- Add day wraparound at midnight edge case test (Hoshe #7)
- Add day() calculation test (Hoshe #7)
- Add out-of-order input rejection test (Hoshe #7)
- Total: 15 → 20 tests

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

server/tests/serialization.rs

@@ -49,3 +49,58 @@ fn empty_snapshot_roundtrip() {
     assert_eq!(decoded.tick, 0);
     assert!(decoded.entities.is_empty());
 }
+
+/// All PlayerAction variants must survive MessagePack round-trip (D-030 Layer 1)
+#[test]
+fn all_player_action_variants_roundtrip() {
+    let actions = vec![
+        PlayerAction::MoveNorth,
+        PlayerAction::MoveSouth,
+        PlayerAction::MoveEast,
+        PlayerAction::MoveWest,
+        PlayerAction::Interact,
+        PlayerAction::UsePerceptionMode("thermal".to_string()),
+        PlayerAction::Pause,
+        PlayerAction::Unpause,
+    ];
+
+    for action in actions {
+        let input = PlayerInput {
+            tick: 1,
+            action: action.clone(),
+        };
+        let bytes = rmp_serde::to_vec(&input).expect("serialize");
+        let decoded: PlayerInput = rmp_serde::from_slice(&bytes).expect("deserialize");
+        assert_eq!(decoded.tick, 1);
+        // Verify the variant survived by re-serializing and comparing bytes
+        let re_bytes = rmp_serde::to_vec(&decoded).expect("re-serialize");
+        assert_eq!(bytes, re_bytes, "round-trip mismatch for action variant");
+    }
+}
+
+/// All EntityKind variants must survive MessagePack round-trip (D-030 Layer 1)
+#[test]
+fn all_entity_kind_variants_roundtrip() {
+    let kinds = vec![EntityKind::Npc, EntityKind::Object, EntityKind::Terrain];
+
+    for (i, kind) in kinds.into_iter().enumerate() {
+        let entity = VisibleEntity {
+            entity_id: i as u64,
+            x: 0.0,
+            y: 0.0,
+            z: 0,
+            kind,
+        };
+        let snapshot = ObserverSnapshot {
+            tick: 0,
+            entities: vec![entity],
+        };
+        let bytes = rmp_serde::to_vec(&snapshot).expect("serialize");
+        let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");
+        let re_bytes = rmp_serde::to_vec(&decoded).expect("re-serialize");
+        assert_eq!(
+            bytes, re_bytes,
+            "round-trip mismatch for EntityKind variant"
+        );
+    }
+}

server/tests/smoke.rs

@@ -1,8 +1,8 @@
 //! Smoke test: the simulation world boots and can run a single tick.
 
 use bevy_app::prelude::*;
-use settled_reach_server::simulation::SimulationPlugin;
 use settled_reach_server::simulation::time::SimulationTime;
+use settled_reach_server::simulation::SimulationPlugin;
 
 #[test]
 fn world_boots_and_ticks() {