Build XCUI splitter foundation and test harness

tests/UI/Editor/integration/input/pointer_states/README.md

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+# Pointer States Validation
+
+可执行 target：
+
+- `editor_ui_input_pointer_states_validation`
+
+运行：
+
+```bash
+build\tests\UI\Editor\integration\input\pointer_states\Debug\XCUIEditorInputPointerStatesValidation.exe
+```
+
+检查点：
+
+1. hover 左侧按钮，只应变化 `hover`
+2. 按住中间按钮，应看到 `focus`、`active`、`capture`
+3. 拖到右侧再松开，应看到 `capture` 清空，route 转到新的目标