[backfill, May 19-20] Undo/redo stack and modal transforms. Dual-mode undo: position-only snapshots for fast grab/scale/rotate restore, full topology serialization for mesh-changing ops. 64-deep stack, bidirectional redo. Grab uses ray-plane intersection for exact cursor tracking. Scale and rotate from mouse deltaX. Extrude wired up (E) auto-entering grab. Delete, subdivide, inset, flip normals, add cube all operational.