Reuse the shared PolygonEditor for slab boundary editing with levelY
and surfaceHeight support. Handles render as 3D cylinders that adapt
to slab thickness. Prevent deselection after drag by suppressing
grid:click propagation, and reset preview state on undo/redo.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
7a871be792