DC.119 BimCraft: add a level below the lowest level

Asked 2026-11-04: is there a way to create a level below the lowest one made so far. Not checked in the code. Do: check whether a negative-elevation level can be added (basement, footings); if not, add Level below to the level controls with undo.

bim-fix-09 2026-11-05: Checked first: no way existed (Add a level on top only; Add levels... takes heights from 0 up; typing a negative elevation moves every level above with it). Built: Levels panel > Add a level below (model-ops newLevelBelow): a level under the lowest, as tall as it, at a negative elevation, named Level -1, Level -2; nothing else moves; one Undo step; redo keeps the same GlobalId; walls can be drawn on it. Tests: node bin/test-bimcraft.mjs --levelbelow, tools/bimcraft-test/browser-model.mjs --only bm2. Help: draw-walls, site-building-and-levels.