Split new_editor compat sources into static library

docs/plan/XCUI_Phase_Status_2026-04-05.md

@@ -30,6 +30,9 @@ Old `editor` replacement is explicitly out of scope for this phase.
 - The default native shell path now also has a cleaner translation-unit seam:
   - legacy ImGui shell chrome / HUD rendering now lives in a dedicated legacy-only `Application` translation unit instead of keeping direct `ImGui::*` calls inside the main native host TU
   - `Application.cpp` no longer directly includes `<imgui.h>`, even though the compatibility host path is still compiled into `new_editor`
+- The `new_editor` build now also has an explicit compatibility-source slice:
+  - legacy ImGui shell sources and vendored ImGui backend sources are now grouped into a dedicated compatibility static library instead of being compiled directly as part of the main `XCNewEditor` source list
+  - the main `XCNewEditor` target still sees ImGui headers through older editor bridge headers, but its default implementation source list is now narrower than the compatibility source list
 - Old `editor` replacement remains deferred; all active execution still stays inside XCUI shared code and `new_editor`.
 
 ## Three-Layer Status
@@ -282,6 +285,9 @@ Current gap:
 - Legacy shell chrome / HUD rendering is now split out of the main `Application.cpp` translation unit:
   - the direct `ImGui::*` shell rendering path now lives in a dedicated legacy-only `Application` implementation file
   - the main `Application.cpp` native host path no longer directly includes `<imgui.h>`, reducing default-path compile-time coupling before the larger compat-target split
+- `new_editor` build composition is now split into main/native and compatibility slices:
+  - the main `XCNewEditor` target no longer compiles legacy ImGui shell/panel/backend source files directly
+  - legacy ImGui shell/panel/backend sources plus vendored ImGui sources now build behind a dedicated compatibility static library that the main executable links
 
 ## Phase Risks Still Open