D9.1 BimCraft: change several columns at once (height, top, size)

Ask: "select multiple columns and edit their height in one go; the height options only show when one column is selected."
Several columns picked together already shared Shape, Size, Top and Height, but a pick that also caught a wall or another element (columns stand in walls on grid lines; a box catches both) showed only "N elements picked" with no fields.
Built: a pick of several kinds lists each kind with "Keep only these", one click keeps the columns and their shared fields show. Fields where the columns differ are empty ("Mixed"), never the first column's value; Height typed once makes every picked column that tall (a column following the level above gets the offset that makes it so); Top keeps each column's own level above.
Files: bimcraft.js showMany, model-grids-ui.js props. Tests: browser-draw06 --only d9.