gui_toolkit 0.40.1

Added

• ui.building(): true between begin_build and end_build (on this thread, whatever the state). An app whose window procedure makes frames (on_frame, inside a loop of the system) asks it before building one.

Fixed

• A build begun inside another one is refused with a clear assertion (ui: begin_build inside another build) instead of breaking the one in progress: begin_build emptied its parent stack, freed its frame arena and selected another state under it, and the outer build failed later at an unrelated place (Wren's crash of 2026-10-26: a bounds check in ui.spacer, after a TrackPopupMenu opened during the build painted the window from WM_PAINT). Test: tests/ui_robust nested_build_test.