Extract XCUI selection model and layout lab click selection

docs/plan/XCUI_Phase_Status_2026-04-05.md

@@ -32,6 +32,7 @@ Old `editor` replacement is explicitly out of scope for this phase.
 - Shared text-editing primitives now live under `engine/include/XCEngine/UI/Text` and `engine/src/UI/Text`, so UTF-8 caret movement, line splitting, and multiline navigation are no longer trapped inside `XCUI Demo`.
 - Shared text-input controller/state now also lives under `engine/include/XCEngine/UI/Text` and `engine/src/UI/Text`, so character insertion, backspace/delete, submit, and multiline key handling no longer need to be reimplemented per host.
 - Shared editor collection primitive classification and metric helpers now also live under `engine/include/XCEngine/UI/Widgets` and `engine/src/UI/Widgets`, covering the current `ScrollView` / `TreeView` / `ListView` / `PropertySection` / `FieldRow` prototype taxonomy.
+- Shared single-selection state now also lives under `engine/include/XCEngine/UI/Widgets` and `engine/src/UI/Widgets` as `UISelectionModel`, so collection-style widget selection no longer has to stay private to `LayoutLab`.
 - Core regression coverage now includes `UIContext`, layout, style, runtime screen player/system, and real document-host tests through `core_ui_tests`.
 
 Current gap:
@@ -66,6 +67,7 @@ Current gap:
 - `LayoutLab` now includes a `ScrollView` prototype and a more editor-like three-column authored layout.
 - `LayoutLab` now also covers editor-facing widget prototypes: `TreeView`, `TreeItem`, `ListView`, `ListItem`, `PropertySection`, and `FieldRow`.
 - `LayoutLab` now consumes the shared `UIEditorCollectionPrimitives` helper layer for collection-widget tag classification, clipping flags, and default metric resolution instead of keeping that taxonomy private to the sandbox runtime.
+- `LayoutLab` now also consumes the shared `UISelectionModel` for click-selection persistence across collection-style widgets, and the diagnostics panel now exposes both hovered and selected element ids.
 - Panel diagnostics were expanded to clearly separate preview/runtime/input state and native vs legacy paths.
 - The editor bridge layer now has smoke coverage for swapchain after-UI rendering hooks and SRV-backed ImGui texture descriptor registration.
 - `Application` no longer owns the ImGui backend directly; window presentation now routes through `IWindowUICompositor` with an `ImGuiWindowUICompositor` implementation, which currently delegates to `IEditorHostCompositor` / `ImGuiHostCompositor`.
@@ -78,17 +80,17 @@ Current gap:
 
 - The shell is still ImGui-hosted.
 - Legacy hosted preview still depends on an active ImGui window context for inline presentation.
-- Editor-specialized widgets are still incomplete at the shared-module level: the authored prototypes exist, but virtualization, selection models, command routing, property editing models, toolbar/menu chrome, and icon-atlas widgets are not yet extracted into reusable XCUI modules.
+- Editor-specialized widgets are still incomplete at the shared-module level: the authored prototypes exist, but virtualization, multi-selection/tree expansion state, command routing, property editing models, toolbar/menu chrome, and icon-atlas widgets are not yet extracted into reusable XCUI modules.
 
 ## Validated This Phase
 
 - `new_editor_xcui_demo_runtime_tests`: `7/7`
-- `new_editor_xcui_layout_lab_runtime_tests`: `6/6`
+- `new_editor_xcui_layout_lab_runtime_tests`: `7/7`
 - `new_editor_xcui_rhi_command_compiler_tests`: `6/6`
 - `new_editor_xcui_rhi_render_backend_tests`: `5/5`
 - `new_editor_xcui_hosted_preview_presenter_tests`: `12/12`
 - `XCNewEditor` Debug target builds successfully
-- `core_ui_tests`: `28/28`
+- `core_ui_tests`: `30/30`
 - `scene_tests`: `65/65`
 - `core_ui_style_tests`: `5/5`
 - `ui_resource_tests`: `11/11`
@@ -101,6 +103,7 @@ Current gap:
 - Common-core `UITextEditing` extraction now owns UTF-8 offset stepping, codepoint counting, line splitting, and vertical caret motion with dedicated `core_ui_tests` coverage.
 - Common-core `UITextInputController` extraction now owns per-field text state, character insertion, enter-submit, and multiline keyboard editing behavior with dedicated `core_ui_tests` coverage.
 - Common-core `UIEditorCollectionPrimitives` extraction now owns the editor collection tag taxonomy and default metric resolution used by current `LayoutLab` widget prototypes, with dedicated `core_ui_tests` coverage.
+- Common-core `UISelectionModel` extraction now owns reusable single-selection state for collection-style widgets, with dedicated `core_ui_tests` coverage.
 - Demo runtime text editing was extended with:
   - click-to-place caret
   - `Delete` support
@@ -109,6 +112,7 @@ Current gap:
 - Demo authored resources updated to exercise the input field.
 - LayoutLab `ScrollView` prototype with clipping and hover rejection outside clipped content.
 - LayoutLab editor-widget prototypes for tree/list/property-style sections with dedicated runtime coverage.
+- LayoutLab click-selection now persists through the shared `UISelectionModel`, including selected-state diagnostics and reusable visual selection feedback on cards, collection rows, and field rows.
 - Schema document support extended with:
   - retained `UISchemaDefinition` data on `UIDocumentModel`
   - artifact schema version bump for UI documents