Build XCUI splitter foundation and test harness

tests/UI/Editor/integration/input/keyboard_focus/View.xcui

@@ -0,0 +1,30 @@
+<View
+  name="EditorInputKeyboardFocus"
+  theme="../../shared/themes/editor_validation.xctheme">
+  <Column padding="24" gap="16">
+    <Card
+      title="Editor Validation | Keyboard Focus"
+      subtitle="当前批次：Tab 焦点遍历 | Enter / Space 激活"
+      tone="accent"
+      height="90">
+      <Column gap="8">
+        <Text text="这是 editor 侧验证场景，不承载 runtime 游戏 UI。" />
+        <Text text="这一轮只检查键盘焦点和激活，不混入复杂 editor 面板。" />
+      </Column>
+    </Card>
+
+    <Card title="Keyboard Focus" subtitle="tab focus active" height="214">
+      <Column gap="12">
+        <Text text="只检查下面三个可聚焦按钮和右下角状态叠层。" />
+        <Row gap="12">
+          <Button id="focus-first" text="First Focus" />
+          <Button id="focus-second" text="Second Focus" />
+          <Button id="focus-third" text="Third Focus" />
+        </Row>
+        <Text text="1. 按 Tab：focus 应依次切到 First / Second / Third。" />
+        <Text text="2. 按 Shift+Tab：focus 应反向切换。" />
+        <Text text="3. focus 停在任一按钮后，按 Enter 或 Space：active 应出现；松开后 active 清空。" />
+      </Column>
+    </Card>
+  </Column>
+</View>