Extract XCUI text editing core and window seam headers

docs/plan/XCUI_Phase_Status_2026-04-05.md

@@ -15,7 +15,7 @@ Old `editor` replacement is explicitly out of scope for this phase.
   - engine runtime coverage was tightened again around `UISystem` and concrete document-host rendering
   - `LayoutLab` continues as the editor widget proving ground for tree/list/property-section style controls
   - the demo sandbox and editor bridge APIs were tightened again without touching the old editor replacement scope
-  - `new_editor` now has an explicit window-level compositor seam through `IEditorHostCompositor` / `ImGuiHostCompositor`
+  - `new_editor` now has an explicit two-step window compositor seam through `IWindowUICompositor` / `ImGuiWindowUICompositor`, layered on top of `IEditorHostCompositor` / `ImGuiHostCompositor`
 - Old `editor` replacement remains deferred; all active execution still stays inside XCUI shared code and `new_editor`.
 
 ## Three-Layer Status
@@ -29,13 +29,14 @@ Old `editor` replacement is explicitly out of scope for this phase.
 - Build-system hardening for MSVC/PDB output paths has started in root CMake, `engine/CMakeLists.txt`, `new_editor/CMakeLists.txt`, and `tests/NewEditor/CMakeLists.txt`.
 - Shared engine-side XCUI runtime scaffolding is now present under `engine/include/XCEngine/UI/Runtime` and `engine/src/UI/Runtime`.
 - Shared engine-side `UIDocumentScreenHost` now compiles `.xcui` / `.xctheme` screen documents into a runtime-facing document host path instead of leaving all document ownership in `new_editor`.
+- 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`.
 - Core regression coverage now includes `UIContext`, layout, style, runtime screen player/system, and real document-host tests through `core_ui_tests`.
 
 Current gap:
 
 - Minimal schema self-definition support is landed, including consistency checks for enum/document-only schema metadata, but schema-driven validation for `.xcui` / `.xctheme` instances is still not implemented.
 - Shared widget/runtime instantiation is still thin and mostly editor-side.
-- Common widget primitives are still incomplete: multiline text editing, tree/list virtualization, property-grid composition, and native image/source-rect level APIs.
+- Common widget primitives are still incomplete: reusable text-input state/view composition on top of the new text helpers, tree/list virtualization, property-grid composition, and native image/source-rect level APIs.
 
 ### 2. Runtime/Game Layer
 
@@ -60,7 +61,7 @@ Current gap:
 - `LayoutLab` now also covers editor-facing widget prototypes: `TreeView`, `TreeItem`, `ListView`, `ListItem`, `PropertySection`, and `FieldRow`.
 - 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 `IEditorHostCompositor` with an `ImGuiHostCompositor` implementation.
+- `Application` no longer owns the ImGui backend directly; window presentation now routes through `IWindowUICompositor` with an `ImGuiWindowUICompositor` implementation, which currently delegates to `IEditorHostCompositor` / `ImGuiHostCompositor`.
 - `XCNewEditor` builds successfully to `build/new_editor/bin/Debug/XCNewEditor.exe`.
 
 Current gap:
@@ -75,7 +76,7 @@ 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`: `16/16`
+- `core_ui_tests`: `21/21`
 - `core_ui_style_tests`: `5/5`
 - `ui_resource_tests`: `11/11`
 - `editor_tests` targeted bridge smoke: `3/3`
@@ -84,6 +85,7 @@ Current gap:
 
 - Demo runtime `TextField` with UTF-8 text insertion, caret state, and backspace.
 - Demo runtime multiline `TextArea` path in the sandbox and test coverage for caret movement / multiline input.
+- Common-core `UITextEditing` extraction now owns UTF-8 offset stepping, codepoint counting, line splitting, and vertical caret motion with dedicated `core_ui_tests` coverage.
 - Demo runtime text editing was extended with:
   - click-to-place caret
   - `Delete` support
@@ -119,6 +121,8 @@ Current gap:
   - SRV-view based texture descriptor registration in `ImGuiBackendBridge`
   - smoke tests for window renderer, ImGui backend bridge, and console sink registration
 - `new_editor` host presentation now has a first-class compositor seam:
+  - `IWindowUICompositor`
+  - `ImGuiWindowUICompositor`
   - `IEditorHostCompositor`
   - `ImGuiHostCompositor`
   - `Application` frame/present flow routed through the compositor instead of direct `m_imguiBackend` ownership