Decouple XCEditorCore from editor app target

docs/plan/editor-core-refactor-plan.md

@@ -105,6 +105,10 @@ Completed boundary cuts:
   explicit app module include roots. It restores project panel, scene viewport,
   viewport render-plan, viewport object-id picker, and app input-routing tests
   without widening the include surface.
+- `XCEditorCore` is now controlled by `XCENGINE_BUILD_XCUI_EDITOR_CORE`
+  instead of the executable-host switch. `XCENGINE_BUILD_XCUI_EDITOR_APP=OFF`
+  can still build the product core and the app-facing test targets when
+  renderer editor support is enabled.
 - The old Win32 tab-drag-drop target test now covers the current reusable
   `XCEditor/Docking/UIEditorDockHostTransfer.h` API through
   `editor_windowing_phase1_tests`.
@@ -280,6 +284,8 @@ Completed follow-up:
 
 ## Phase 2: Introduce XCEditorCore
 
+Status: completed and build-graph enforced.
+
 Create:
 
 ```cmake
@@ -316,6 +322,19 @@ Important: do not hide Win32/D3D12 in `XCEditorCore` just to make the first
 CMake edit easier. If a source file needs `windows.h`, it belongs in the host
 side until a neutral interface exists.
 
+Completed target-graph cut:
+
+- `XCENGINE_BUILD_XCUI_EDITOR_CORE` controls `XCEditorCore` and defaults to
+  `ON`.
+- `XCENGINE_BUILD_XCUI_EDITOR_APP` controls only the concrete executable host
+  target `XCEditor`.
+- `XCEditor` requires `XCENGINE_BUILD_XCUI_EDITOR_CORE=ON`.
+- When either the core library or executable host is enabled, the root build
+  defaults `XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON` because editor core
+  owns viewport services that link renderer editor support.
+- The host-off validation shape is:
+  `-DXCENGINE_BUILD_XCUI_EDITOR_APP=OFF -DXCENGINE_BUILD_XCUI_EDITOR_CORE=ON -DXCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON`.
+
 ## Phase 3: Restore App-Core And App Feature Tests
 
 Status: completed for the initial app-core and app feature/viewport suites.
@@ -512,6 +531,16 @@ cmake --build build --config Debug --target editor_app_feature_tests
 cmake --build build --config Debug --target editor_windowing_phase1_tests
 ```
 
+To prove the product core boundary without the executable host, configure a
+separate build directory with:
+
+```powershell
+cmake -S . -B build/editor_core_hostoff -DXCENGINE_BUILD_XCUI_EDITOR_APP=OFF -DXCENGINE_BUILD_XCUI_EDITOR_CORE=ON -DXCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON
+cmake --build build/editor_core_hostoff --config Debug --target XCEditorCore
+cmake --build build/editor_core_hostoff --config Debug --target editor_app_core_tests
+cmake --build build/editor_core_hostoff --config Debug --target editor_app_feature_tests
+```
+
 When app smoke is available:
 
 ```powershell