feat(simulation): #523 add zone_id to ObserverSnapshot (D-077, protocol v11)

Server-tracked zone_id on VisibleTile for D-073 zone crossfade and
D-059 deep fog temperature tint. ZoneMap resource backed by BTreeMap,
observer enrichment in snapshot assembly. Backwards-compatible: v10
payloads deserialize with zone_id: None.

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

server/tests/gen_fixtures.rs

@@ -188,6 +188,7 @@ fn generate_msgpack_fixtures() {
                 z: 0,
                 visibility: VisibilitySector::Forward,
                 tile_kind: TileKind::Floor,
+                zone_id: Some(1),
             },
             VisibleTile {
                 x: 11,
@@ -195,6 +196,7 @@ fn generate_msgpack_fixtures() {
                 z: 0,
                 visibility: VisibilitySector::Peripheral,
                 tile_kind: TileKind::Floor,
+                zone_id: Some(1),
             },
             VisibleTile {
                 x: 10,
@@ -202,6 +204,7 @@ fn generate_msgpack_fixtures() {
                 z: 0,
                 visibility: VisibilitySector::Forward,
                 tile_kind: TileKind::Floor,
+                zone_id: None,
             },
         ],
         nearby_interactions: vec![],

server/tests/serialization.rs

@@ -241,6 +241,7 @@ fn snapshot_v2_fields_roundtrip() {
                 z: 0,
                 visibility: VisibilitySector::Forward,
                 tile_kind: TileKind::Floor,
+                zone_id: None,
             },
             VisibleTile {
                 x: 6,
@@ -248,6 +249,7 @@ fn snapshot_v2_fields_roundtrip() {
                 z: 0,
                 visibility: VisibilitySector::Peripheral,
                 tile_kind: TileKind::Wall,
+                zone_id: None,
             },
         ],
         nearby_interactions: vec![],
@@ -312,7 +314,7 @@ fn protocol_version_constant_matches_snapshot() {
     let snapshot = test_snapshot(0, vec![]);
     assert_eq!(snapshot.version, PROTOCOL_VERSION);
     assert_eq!(
-        PROTOCOL_VERSION, 10,
+        PROTOCOL_VERSION, 11,
         "bump this assertion when protocol version changes"
     );
 }
@@ -862,6 +864,7 @@ fn boundary_value_in_tile_position() {
             z: 0,
             visibility: VisibilitySector::Forward,
             tile_kind: TileKind::Floor,
+            zone_id: None,
         }];
         let bytes = rmp_serde::to_vec_named(&snapshot)
             .unwrap_or_else(|e| panic!("encode tile x/y={} failed: {}", tile_val, e));
@@ -1264,6 +1267,83 @@ fn v9_payload_deserializes_into_v10_struct() {
     );
 }
 
+/// v10 payload (without zone_id on VisibleTile) deserializes into the v11 struct
+/// via #[serde(default)]. Guards backwards compat during migration (#523, D-077).
+#[test]
+fn v10_payload_deserializes_into_v11_struct() {
+    // V10 VisibleTile: no zone_id field
+    #[derive(serde::Serialize)]
+    struct VisibleTileV10 {
+        x: i32,
+        y: i32,
+        z: i32,
+        visibility: VisibilitySector,
+        tile_kind: TileKind,
+    }
+
+    #[derive(serde::Serialize)]
+    struct ObserverSnapshotV10 {
+        version: u8,
+        tick: u64,
+        game_time: GameTime,
+        player_facing: FacingDirection,
+        player_stance: MovementStance,
+        player_inventory: Vec<InventoryItem>,
+        entities: Vec<VisibleEntity>,
+        visible_tiles: Vec<VisibleTileV10>,
+        nearby_interactions: Vec<NearbyInteraction>,
+        current_monologue: Option<MonologueEvent>,
+        pending_recognitions: Vec<PendingRecognitionWire>,
+        dialogue_response: Option<DialogueResponseEvent>,
+        blocked_entities: Vec<u64>,
+        scan_events: Vec<settled_reach_server::simulation::contraband::ScanEvent>,
+        sound_events: Vec<settled_reach_server::simulation::sound::SoundEvent>,
+        rng_seed: Option<u64>,
+    }
+
+    let v10 = ObserverSnapshotV10 {
+        version: 10,
+        tick: 300,
+        game_time: GameTime {
+            day: 0,
+            time_of_day: 0,
+            day_phase: DayPhase::Morning,
+            tick_rate: TickRate::Full,
+        },
+        player_facing: FacingDirection::North,
+        player_stance: MovementStance::Walk,
+        player_inventory: vec![],
+        entities: vec![],
+        visible_tiles: vec![VisibleTileV10 {
+            x: 5,
+            y: 10,
+            z: 0,
+            visibility: VisibilitySector::Forward,
+            tile_kind: TileKind::Floor,
+        }],
+        nearby_interactions: vec![],
+        current_monologue: None,
+        pending_recognitions: vec![],
+        dialogue_response: None,
+        blocked_entities: vec![],
+        scan_events: vec![],
+        sound_events: vec![],
+        rng_seed: Some(42),
+    };
+
+    let bytes = rmp_serde::to_vec_named(&v10).expect("serialize v10");
+    let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes)
+        .expect("v10 payload should deserialize into v11 struct via serde(default)");
+
+    assert_eq!(decoded.version, 10, "version field preserved from v10");
+    assert_eq!(decoded.tick, 300);
+    assert_eq!(decoded.visible_tiles.len(), 1);
+    assert_eq!(
+        decoded.visible_tiles[0].zone_id, None,
+        "missing zone_id should default to None"
+    );
+}
+
 /// NearbyInteraction.object_type round-trips through MessagePack (#422).
 /// Verifies object_type=Some(Container) survives the wire.
 #[test]