Build XCUI splitter foundation and test harness

new_editor/src/editor/EditorShellAsset.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <filesystem>
+#include <string>
+
+namespace XCEngine::NewEditor {
+
+struct EditorShellAsset {
+    std::string screenId = "new_editor.shell";
+    std::filesystem::path documentPath = {};
+    std::filesystem::path themePath = {};
+    std::filesystem::path captureRootPath = {};
+};
+
+EditorShellAsset BuildDefaultEditorShellAsset(const std::filesystem::path& repoRoot);
+
+} // namespace XCEngine::NewEditor