Add XCUI runtime screen stack helper

docs/plan/XCUI_Phase_Status_2026-04-05.md

@@ -73,7 +73,7 @@ Current gap:
 - `new_editor_xcui_rhi_command_compiler_tests`: `6/6`
 - `new_editor_xcui_rhi_render_backend_tests`: `5/5`
 - `XCNewEditor` Debug target builds successfully
-- `core_ui_tests`: `14/14`
+- `core_ui_tests`: `16/16`
 - `core_ui_style_tests`: `5/5`
 - `ui_resource_tests`: `11/11`
 - `editor_tests` targeted bridge smoke: `3/3`
@@ -102,8 +102,10 @@ Current gap:
 - Engine runtime layer added:
   - `UIScreenPlayer`
   - `UIDocumentScreenHost`
+  - `UIScreenStackController`
   - `UISystem`
   - layered screen composition and modal blocking semantics
+- Runtime/game integration scaffolding now includes reusable `HUD/menu/modal` stack helpers on top of `UISystem`.
 - Runtime document-host draw emission now preserves button labels for shared screen rendering.
 - RHI image path improvements:
   - clipped image UV adjustment