fix splitter filters being lost when rotating in place and add test

src/test/BuildingTest.cpp

@@ -1427,6 +1427,36 @@ TEST_CASE("BuildingSystem: rotateInPlace re-registers a belt tile with BeltSyste
     REQUIRE(belts.tryPutItem(QPoint(0, 0), makeItem("iron_ore")));
 }
 
+TEST_CASE("BuildingSystem: rotateInPlace preserves the output filters of a splitter "
+          "(REQ-BLD-SPLITTER)", "[building][rotate-in-place]")
+{
+    PlacementFixture f;
+
+    const QPoint tile(5, 5);
+    const BuildingId id = f.bs.place(BuildingType::Splitter, tile, Rotation::East, 0).value();
+
+    // Run until construction completes, so the splitter is registered with BeltSystem.
+    Tick tick = 0;
+    while (f.bs.getAllBuildings().empty() && tick < 100000)
+    {
+        runTicks(f.bs, f.belts, 1, tick);
+    }
+    REQUIRE(f.bs.getAllBuildings().size() == 1);
+
+    const std::vector<ItemType> filterA{ ItemType{"iron_ore"} };
+    const std::vector<ItemType> filterB{ ItemType{"copper_ore"} };
+    f.belts.setSplitterFilters(tile, filterA, filterB);
+
+    f.bs.rotateInPlace(id, Rotation::North);
+
+    // The tile is re-registered with BeltSystem carrying the filters it had before
+    // the rotation — rotating must not reset a configured splitter to "accept all".
+    const std::optional<BeltSystem::SplitterInfo> info = f.belts.getSplitterInfo(tile);
+    REQUIRE(info.has_value());
+    REQUIRE(info->filterA == filterA);
+    REQUIRE(info->filterB == filterB);
+}
+
 TEST_CASE("BuildingSystem: splitter filters configured on a construction site carry over "
           "to the built splitter (REQ-BLD-SITE-CONFIG)", "[building]")
 {