[saworbit]

The capture's structure shot could not hold a selected piece. _build_structures returns generator ids, which are Strings, and the earlier tries handed one to _select_only(). That cleared the selection, then EditorSelection.add_node() refused the String and the shot went out with nothing selected. Nothing in the plugin cleared it.

• New ui_structure_piece shot: a tread of the spiral staircase selected, the Structure section on Update with Detach beside it, and the context toolbar on one row ending in Upd and Det. _structure_piece() turns a generator id into its highest piece, so the gizmo stands on something above the floor.
• The shot calls plugin.update_overlays() before capturing. Without it the toolbar keeps its old width and stands one button wide down the viewport; filed as [Viewport] The context toolbar stays one button wide after a selection made outside the viewport #1375.
• ui_structure.png is unchanged. The new image goes under "Changing a structure after you have built one" in the User Guide and in the shot table in docs/demos/README.md.

Checked: capture_ui.py from the branch (the new image looked at, every other image checked out again), mkdocs build --strict, CI's two lint commands, check_script_warnings.py, uid parity, test_mcp_listener (loads the docshot) 33/33.

Fixes #1370