Build XCUI splitter foundation and test harness

tests/UI/Runtime/unit/test_ui_runtime_splitter_validation.cpp

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+#include <gtest/gtest.h>
+
+#include <XCEngine/UI/Runtime/UIScreenDocumentHost.h>
+#include <XCEngine/UI/Runtime/UIScreenPlayer.h>
+
+#include <chrono>
+#include <filesystem>
+#include <fstream>
+#include <string>
+
+namespace {
+
+using XCEngine::UI::UIRect;
+using XCEngine::UI::Runtime::UIScreenAsset;
+using XCEngine::UI::Runtime::UIScreenFrameInput;
+using XCEngine::UI::Runtime::UIScreenPlayer;
+using XCEngine::UI::Runtime::UIDocumentScreenHost;
+
+namespace fs = std::filesystem;
+
+class TempFileScope {
+public:
+    TempFileScope(std::string stem, std::string extension, std::string contents) {
+        const auto uniqueId = std::to_string(
+            std::chrono::steady_clock::now().time_since_epoch().count());
+        m_path = fs::temp_directory_path() / (std::move(stem) + "_" + uniqueId + std::move(extension));
+        std::ofstream output(m_path, std::ios::binary | std::ios::trunc);
+        output << contents;
+    }
+
+    ~TempFileScope() {
+        std::error_code ec;
+        fs::remove(m_path, ec);
+    }
+
+    const fs::path& Path() const {
+        return m_path;
+    }
+
+private:
+    fs::path m_path = {};
+};
+
+UIScreenAsset BuildScreenAsset(const fs::path& viewPath, const char* screenId) {
+    UIScreenAsset screen = {};
+    screen.screenId = screenId;
+    screen.documentPath = viewPath.string();
+    return screen;
+}
+
+UIScreenFrameInput BuildInputState(std::uint64_t frameIndex = 1u) {
+    UIScreenFrameInput input = {};
+    input.viewportRect = UIRect(0.0f, 0.0f, 960.0f, 720.0f);
+    input.frameIndex = frameIndex;
+    input.focused = true;
+    return input;
+}
+
+} // namespace
+
+TEST(UIRuntimeSplitterValidationTest, InvalidSplitterArityProducesExplicitFrameError) {
+    TempFileScope viewFile(
+        "xcui_runtime_invalid_splitter",
+        ".xcui",
+        "<View name=\"Invalid Splitter Test\">\n"
+        "  <Column padding=\"16\" gap=\"10\">\n"
+        "    <Splitter id=\"broken-splitter\" axis=\"horizontal\" splitRatio=\"0.5\">\n"
+        "      <Card title=\"Only Child\" />\n"
+        "    </Splitter>\n"
+        "  </Column>\n"
+        "</View>\n");
+
+    UIDocumentScreenHost host = {};
+    UIScreenPlayer player(host);
+
+    ASSERT_TRUE(player.Load(BuildScreenAsset(viewFile.Path(), "runtime.invalid_splitter")));
+
+    const auto& frame = player.Update(BuildInputState());
+    EXPECT_FALSE(frame.errorMessage.empty());
+    EXPECT_NE(frame.errorMessage.find("broken-splitter"), std::string::npos);
+    EXPECT_NE(frame.errorMessage.find("exactly 2 child elements"), std::string::npos);
+}