Build XCUI splitter foundation and test harness

tests/UI/Editor/integration/README.md

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+# Editor UI Integration Validation
+
+This directory contains the manual XCUI validation system for editor-facing scenarios.
+
+Structure:
+
+- `shared/`: shared host, native renderer, screenshot helper, scenario registry
+- `input/`: input-related validation category
+- `layout/`: layout and shell-foundation validation category
+
+Rules:
+
+- One scenario directory maps to one executable.
+- Do not accumulate unrelated checks into one monolithic app.
+- Shared infrastructure belongs in `shared/`, not duplicated per scenario.
+- Screenshots are stored per scenario inside that scenario's `captures/` folder.
+
+Build:
+
+```bash
+cmake --build build --config Debug --target editor_ui_integration_tests
+```