Build XCUI splitter foundation and test harness

new_editor/include/XCNewEditor/Widgets/UIEditorCollectionPrimitives.h

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+#pragma once
+
+#include <XCEngine/UI/Style/Theme.h>
+
+#include <cstdint>
+#include <string_view>
+
+namespace XCEngine {
+namespace UI {
+namespace Widgets {
+
+enum class UIEditorCollectionPrimitiveKind : std::uint8_t {
+    None = 0,
+    ScrollView,
+    TreeView,
+    TreeItem,
+    ListView,
+    ListItem,
+    PropertySection,
+    FieldRow
+};
+
+UIEditorCollectionPrimitiveKind ClassifyUIEditorCollectionPrimitive(std::string_view tagName);
+bool IsUIEditorCollectionPrimitiveContainer(UIEditorCollectionPrimitiveKind kind);
+bool UsesUIEditorCollectionPrimitiveColumnLayout(UIEditorCollectionPrimitiveKind kind);
+bool IsUIEditorCollectionPrimitiveHoverable(UIEditorCollectionPrimitiveKind kind);
+bool DoesUIEditorCollectionPrimitiveClipChildren(UIEditorCollectionPrimitiveKind kind);
+float ResolveUIEditorCollectionPrimitivePadding(
+    UIEditorCollectionPrimitiveKind kind,
+    const Style::UITheme& theme);
+float ResolveUIEditorCollectionPrimitiveDefaultHeight(
+    UIEditorCollectionPrimitiveKind kind,
+    const Style::UITheme& theme);
+float ResolveUIEditorCollectionPrimitiveIndent(
+    UIEditorCollectionPrimitiveKind kind,
+    const Style::UITheme& theme,
+    float indentLevel);
+
+} // namespace Widgets
+} // namespace UI
+} // namespace XCEngine